According to Pollster.com, Romney is currently up by about six points on Giuliani in New Hampshire and 14 over McCain. (Huckabee's about 22 points back, though it's important to note that the Pollster.com chart is a bit of a lagging indicator. He could be a little closer by now.) As long as neither Giuliani nor McCain makes much a showing in Iowa (as appears likely), I think a Romney victory there--even one that doesn't generate a huge media splash--basically reinforces the status quo in New Hampshire, which means Romney wins fairly comfortably there, too. And if Romney wins both Iowa and New Hampshire by reasonable margins, things start to look very good for him.
Or, put differently, Giuliani needs either a close second in New Hampshire or an outright victory there to slow Romney's momentum. If Romney finishes first and Giuliani a distant third or fourth in Iowa, I don't see Giuliani turning around and gaining much ground on Romney in New Hampshire, rergardless of what Huckabee does to Romney's media coverage heading out of Iowa.
But what if Rudy makes a major push in New Hampshire? Up until now, he hasn't done that. But it looks like that might be changing. He's spending four straight days there this week. If Rudy starts rolling in New Hampshire, Romney is definitely going to need real momentum coming out of Iowa.
Update: I'd missed this Politico story about how Rudy's now planning to make a major play for New Hampshire. Meanwhile, in comments, Derek Catsam argues that Romney's lead in New Hampshire may be because New Hampshire voters know him best, since Massachusetts is right next door. But seeing as how Romney is no longer that popular in Massachusetts--this April poll had Rudy beating Romney by 12 points in Romney's home state--I don't know if that's the explanation. Rather, I think Romney's lead in New Hampshire is attributable to the fact that he's been there a ton and has been on the air there constantly. Now that Rudy looks like he's going to match Mitt on those scores, I think Romney's lead may no longer be so safe.