Political junkies, e.g., people who write for and comment at political blogs, need to be careful not to assume people remember the ins and outs of the 2008 campaign. The number of people who would definitely not vote for Romney in a general election has dropped significantly since 2007. The WaPo asserts this is because Republicans have warmed to him, when the real answer may be they have forgotten about him, or never knew much about him. In the latter case, some aggressively comparative campaigning might erode the primary pillar of the Romney campaign, electability.
On the other hand, I agree with Bryan that it would be unwise to attack Cain generally, and not just because Cain is likeable. Rather, the point for Perry (or Cain, for that matter) is to try to become the consensus NotRomney. Attacking other NotRomneys, especially one as likeable as Cain, does not move toward that goal. Although Bryan thinks Bachmann hurt herself by going negative early, she also hurt herself by choosing to make Pawlenty and Perry the focus of her most pointed criticisms.