Nope. Ron Paul's supporters probably won't go anywhere, and they'll get him maybe 20% of the vote at most.
If Romney wins a couple early states they'll fall in line behind Romney IMO.
I agree that these will overwhelmingly go to Romney. Although what these speculations leave out is the largest group of GOP voters of all: Those who haven't even started following the campaign yet, are undecided, or aren't strongly attached to any particular candidate. I strongly suspect most of them will go to Romney as well. It's his turn to be the nominee.
So far I see no reason to suspect that Cain is anything more than the flavor of the week, just as Chris Christie, Rick Perry, Michele Bachmann, Tim Pawlenty, Donald Trump, and Sarah Palin were. As far as I can tell, the best predictor of how much "staying power" a candidate has, is how long he/she has already been a serious contender. Herman Cain has only been taken seriously for about two weeks now...which is about how much longer I expect that to last.