Several commentators have this right: when he tries to give a conservative answer, he really has to think about it. I believe it happens because his first thoughts are liberal.
And this talk about raising taxes on the rich shows that he is firmly in the Kerry/Buffett/Corzine camp of the wealthy economic liberal. Romney has also indicated before that he "can't" go out and talk about cutting taxes on the rich because he is rich, and he believes it would look bad. So he's blatantly doing the same thing liberals do, attacking the rich in order to try to get elected by everyone else. It's just somewhat shocking he can maintain establishment Republican support while doing this, although he did take a noticeable hit the day he talked about indexing the minimum wage to inflation. You could tell the establishment's enthusiasm decreased a bit that day, although it seemed to be on its way back up since then. Maybe this latest news will hurt it again.