Bernstein examines it:
The best hint you can get of Romney's ceiling isn't the horse race head-to-head numbers; it's his unfavorable numbers, which run right now at around 20% of all Republicans (24% of those who recognize his name). But even that has very limited utility; after all, we've just seen Newt Gingrich dramatically improve his favorable/unfavorable ratings, and surely Newt's unfavorables were a lot more deep-seated than Romney's.
The Dish 