Matt has an interesting post comparing Romney's rightward flip-flops to John Edwards' journey to the left over the last few years. He notes that "liberals are primed to believe that Edwards is sincere in his new, more liberal persona, since we tend to think that the New Edwards' stands are correct on the merits," and suggests that presumably the same should go for conservatives and Romney: "Becoming pro-life looks like a pander to me, but to people who find the pro-life view plausible, the view that Romney converted to it is also going to seem more convincing."
I'm largely inclined to agree, and I'd endorse Matt's additional contention that a politician's persona, however calculated, is usually a pretty reliable guide to how he'll govern, regardless of what he "really" believes. That said, I'd suggest a distinction between Romney and Edwards, which may explain why Romney has gotten more grief for his ideological transmogrification - namely, that Edwards' political migration on domestic policy looks more like a development than a flip-flop. In 2004, he was running as a Shrum-style "people against the powerful" tribune of the working and middle classes, whereas today he's running as more of an anti-poverty crusader - and while these are definitely different positions, they're not necessarily contradictory. (Indeed, his way of framing his Shrumian themes - all that "Two Americas" rhetoric - blurs easily into LBJ-style poverty talk.) This contrasts with Romney's simple volte-faces on guns, abortion and immigration, where his earlier position and his new one are mutually exclusive, and where he has to say that he changed his mind, rather than being able to claim that the new position is related somehow to the old one.
(It's also the case that Romney has changed his mind on hot-button issues that the press likes to write about, while Edwards has changed his mind on wonky issues the press finds boring - so that may play into the dynamic as well.)
On foreign policy, of course, Edwards has decidedly flip-flopped, rather than developed, in his views on the Iraq War. But this flip-flop is shared by half the Democratic Party, and while Edwards' leftward turn on the issue happened early enough to look like a political calculation - positioning him to be the anti-Hillary in '08 - it also happened early enough to make him look relatively prescient, which tends to take the sting out of charges of opportunism.