Okay, maybe "big" is a little much. But he has seems to have a clear path, barring a Fred Thompson surge, to winning Iowa. An Iowa victory in a noncompetitive race isn't all that impressive, but neither is it just a native-son victory like Tom Harkin in 1992, so a win will give him at least a slight bump in media attention heading into New Hampshire, where he's certain to be competitive anyway. If he takes New Hampshire, he'll be headed into Florida as a two-time winner, which in a foreshortened primary season will make him the natural rallying point for the anti-Giuliani vote. It's a safe bet, I think, that the race will either be over or down to two candidates after Super Tuesday, and I also think it's a safe bet that Rudy will be one of the final two; Romney's Iowa edge, if he can hold it, makes it that much more likely that he'll be the other man standing at that point. (Though of course the flip side of this is that if loses New Hampshire to anyone except Rudy, he's probably toast.)
I also think Rich Lowry makes a good point (one of several in the post) about Romney and the debates:
I take it for granted Romney is going to be polished and make a good impression, so I may discount it and judge him in a more niggling way. But most voters won't do this.