There's one scholastic problem, of course, with simply identifying SOM with Cartesianism and that's Pirsig's ZMM tracing of the problem to the Greeks. If you look into the historical studies on the subject, they'll usually tell you that there were some significant changes between the two time periods, typically something like the ascendancy of epistemology in Descartes. I think what can be said, though, is that there are a whole host of problems born of errant distinctions and that many of these distinctions orbit each other. If you start with any one of them in a particular orbit, you'll more than likely get weighed down with the others. It is certainly apparent to me that Pirsig shunts a whole host of distinctions under SOM's canopy and his narrative extending back to the Greeks simply helps show how Descartes and his problematic were still working in Greek shadows.
To help see this, I would suggest looking at the way Pirsig lines up Truth and dialectic on one side and the Good and rhetoric on the other. Pirsig's analysis of Plato, I think, is dead on. He says that Plato ostensibly sets the Good as the highest Form, as the highest principle in his system. But if you look closer, the highest principle is subservient to the dialectic because the dialectic is how the Good is discovered. Dialectic, or Method, attains priority over the Good. I would suggest that this analysis gives us the first suggestion that epistemology must have priority over metaphysics, though this became apparent (or at least pressing) to philosophers only after Descartes. I think the historical reasons for this, and its run up to contemporary times, are excellently given by Stephen Toulmin in his Cosmopolis. This passage from the end should have no small amount of resonance with Pirsigians:
Throughout history, the development of philosophy has displayed a sequence of pendulum swings between two rival agendas. On one agenda, the task of philosophy is to analyze all subjects in wholly general terms; on the other, it is to give as general an account as the nature of the field allows. Theoretically minded Platonists speculate freely, framing broad generalizations about human knowledge; practical-minded Aristotelians hesitate to claim universality in advance of actual experience. So read, the move from 16th-century humanism to 17th-century exact science was a swing from the practical, Aristotelian agenda, to a Platonist agenda, aimed at theorectical answers. The dream of 17th-century philosophy and science was Plato's demand for episteme, or theoretical grasp: the facts of 20th-century science and philosophy rest on Aristotle's phronesis, or practical wisdom. When Wittgenstein and Rorty argue that philosophy today is at "the end of the road", they are overdramatizing the situation. The present state of the subject marks the return from a theory-centered conception, dominated by a concern for stability and rigor, to a renewed acceptance of practice, which requires us to adapt action to the special demands of particular occasions.One thing I wish Pirsig would have done was distinguish more sharply between the various problems that were included in his SOM. I think his neglect to do so leads him to conflate materialism (an ontological thesis) with epistemology in a less than helpful way. I think this conflation is what leads him to say, fallaciously, that any philosopher worth his salt forwarded seriously the thesis that values are not real. The materialism comes out when Pirsig identifies objects with the material realm and subjects with the apparitions in our minds. It is only combining this thesis, that the world is nothing but corpuscles bouncing in a void, with the episteomological distinction between objectivity and subjectivity that we get the absurd claim that rocks are real, but our desire for ice cream is not. I don't think any of the great Western philosophers have ever tendered such a suggestion, though vulgar, commonsensical manglings of the great philosophers may generate such a position. But we should be a bit more sophisticated than that. The two are obviously connected in important ways, but not in the way that Pirsig claims. Pirsig aims this charge at the logical positivists and I think this is why Galen Strawson, who should otherwise have at least picked up from his dad, P.F. Strawson, a thing or two about Kantian philosophy and the persuasiveness of a paradigm of thought, says that SOM is a strawman. The positivists claimed that values were cognitively meaningless, which leads to emotivism, not unreal and there is big difference.
I would maintain, then, that the road to modern philosophy (and Descartes in particular) was begun by Plato's attempt to combine Euclid's geometrical method with Socrates' conversational style, or to put it another way, as Rorty says, his attempt to combine the Greek's love of argumentation with their love of wisdom. ("Philosophy as a Kind of Writing") The earlier Greek philosophers had taken the first tentative steps by attempting to assert what was really going on behind the appearences (which Parmenides made explicit). But Plato took the decisive step by arguing that we needed something to decide between all these competing hypotheses. Plato's alteration of Socrates' dialectic (the elenchus) from a searching conversation between several people into a method by which Truth is ascertained was the push needed to solidify Method's grip over philosophy, taking us down the path towards epistemological priority. It only stalled in the bed because Aristotle's philosophy, with its emphasis on practical wisdom, took hold of the Romans' minds, before all of it tumbled into obscurity for a thousand years.
The rise of Descartes marked the rise epistemology. Richard Popkin argues that Descartes, like Plato before him, was involved in a program of cooptation. The patriarch of Renaissance philosophy was Montaigne. He exemplified the skeptical tradition of Pyrrho that had blended with Aristotle's philosophy. Descartes took this skepticism and pushed it towards his own ends. By coopting something like the skeptical devices used by these earlier philosophers, Descartes tried to say something positive. He used it as a method to reach his Archimedean foundation. The actual skeptics during this time were outraged that Descartes would call his philosophy "skeptical." Pyrrhonian skepticism doesn't say anything positive, it only provides the negative point that maybe we shouldn't be trying to say those positive things. They vigorously attacked Descartes, but to no avail. As Toulmin likes to say, "Descartes' coup d'etat" was complete and his program of philosophy captured the imaginations of European intellectuals.
Between Descartes and Locke, modern philosophy saw the rise of what Quine calls the "idea idea," the idea that we have ideas over here in the mind and the material world out there. This was the first step towards divorcing man from the world, observer from the observed, subject from object. Rorty suggests that, if we accept Descartes' representational problematic (as opposed to sticking to Berkeley's dictum "ideas can only represent other ideas"), modern philosophy can be seen as a series of attempts at trying to get the subject and object back together. This is why Pirsig can be seen to have so many prima facie similarities with 19th century philosophers (particularly Hegel), because the "subject/object" idiom was in vogue after Kant put his stamp on philosophy.
However, this idiom never really took that strong a hold in Britain. The beginning of the divorce between Anglophone philosophy and Continental, Franco-German philosophy was with the rise of linguistic philosophy. The triumph of Russell over Anglo-American philosophy was matched by Husserl's triumph over Continental. In the subsequent decades, analytic philosophers began focusing more and more on matters that were dry and remote, stuff that simply seemed "academic" to laypeople, talk about predication, modal logic, etc. It didn't have the world-historical verve and romance that Kant and Hegel gave to Continental philosophy, which continued in mutated form the subject/object idiom. But despite these superficial differences, both idioms have held to an essentially Cartesian picture of philosophy. Some philosophers like Rorty, Robert Brandom, Samuel Wheeler, and Henry Staten among others in the late 20th century, have been able to transcend the differences in idioms and show how paradigmatic philosophers like Donald Davidson and Jacques Derrida are making essentially the same points.
I think by the end of this story we can begin to see why Pirsig's SOM raised so many hackles. I would contend that SOM is the product of Plato and Descartes, but it is the divergence in philosophy since Kant (either towards Hegel or towards Frege) that tells us something about the reception of Pirsig's philosophy. Pirsig is reacting to the logical positivists, a camp of Anglophone philosophers (despite the most famous ones being German), but he assigns the problem dramatic importance, which is entirely Continental in demeaner. This is why we get Galen Strawson going, "What the hell are you talking about?" Its not that SOM as a target misses its mark, or is completely incoherent, its that analytic philosophers can't imagine why Pirsig talks about it like its a world-historical conspiracy. Foucault and other Continental philosophers, however, might have been perfectly happy with the kind of spin Pirsig puts on the story.
In a qualified sense, I think SOM is just another name for Cartesianism, but I also think its just another name for Platonism and Kantianism. The reason is that I think SOM is just one more signifier for a tradition that stretches from the beginning of philosophy to our current stage. The original battle lines were drawn between rhetoric and dialectic, and though the battles have changed over the years, essentially the same war is being fought. One of the great facets about intellectual history is that at every stage in the sequence, we always find a Sophist, or Pyrrhonian skeptic, or pragmatist to counter the Platonist, Cartesian, or Kantian.