He'll finish well, that's virtually assured. Something like 20-25% of the vote. Romney should also get 20-25% of the vote. It's too early to tell if Paul will be first, though. You have this bunch of nobodies splitting the non-Romney vote at the back of the pack - your Bachmanns, Gingriches, Perries, and Santorums. If that non-Romney vote gets their shit together, they'll coalesce around one candidate and that candidate will win - realistically, that candidate will be Santorum or Perry, probably Santorum.
A Ron Paul victory would be funnest of all - he'd have a real shot at New Hampshire, too, but it would end up with Romney winning the nomination. Santorum or Perry winning would be best because they could actually beat Mittens. I'll just about blow my load if Mittens comes in third in Iowa (if you had told him one year ago that he was gonna come in third in Iowa...)