I've always tried to be careful quoting David Frum, mostly because the old "even the liberal XXXX says" trick always struck me as gimmicky. But more than that, my impression of Frum has always been that he disagreed with the conservative movement on process, not goals. This is why his drumming out of the American Enterprise Institute seems so weird. It's not like Frum endorsed the public option; his point was that conservatives should have engaged in hopes of making the bill even more conservative.
Ezra makes a useful distinction between being a conservative think tank (or pundit) and just parroting Republican talking points. It really looks like Frum's rift with the right originates in his unwillingness to do the latter. This is about process not goals. It's worth contrasting Frum with Andrew, who I think differs with the right on core goals. Andrew doesn't simply believe the GOP should compromise he thinks the current health care bill is a good idea. Whereas Frum endorsed John McCain (though he was sharply critical of Sarah Palin), Andrew endorsed Barack Obama. Frum coined the term "Axis of Evil" and wrote a book defending the Iraq War. Andrew supported the war, but now regards his support with regret.
I'm not attacking Andrew's conservatism, as much as I'm making the point that his break is as much about the means Republicans embrace, as it is about the ends they're pursuing. Frum, from what I gather, is just questioning means.