Romney has been leading in Iowa and NH forever and therefore has to be considered the prohibitive favorite. The problem is, McCain, Giuliani and even Thompson haven't been as active in Iowa or really spent anything on television. Giuliani only recently went on the air in New Hampshire and upped his Iowa effort. If Iowa is essentially written off by everyone except Romney and Huckabee it becomes a little more difficult to assess the impact of a Romney win there on the prospects of McCain and Giuliani (Thompson is all but done). Certainly a Huckabee win would be huge, considering the scrappy nature of his campaign and the money Romney has poured into his own effort there. But Romney still has to be considered the prohibitive favorite unless or until Huckabee passes him in Iowa.
On the plus side, there's a good chance the Republican race will be a mess even after Feb 5th. It has the potential to dissolve into a series of nasty fights in a variety of states between different groups of candidates. In all likely hood, the Democratic race will be over after New Hampshire.