Here is one more way to look at the impact of Iowa. The plot shows each poll (tracking polls are included only when their samples don't overlap, e.g. every 3rd day for a 3 day track.) Polls ending on the same day are separated in the chart, though the order is arbitrary within day.
McCain had already captured the lead over Romney by the Iowa caucus. While three post-Iowa polls still find a Romney lead, the majority of polls put McCain in the lead. Moreover, the height of the bars is quite similar before and after Iowa, indicating little effect.
That lack of change is interesting since Romney might have been expected to fall due to his disappointing second place finish in Iowa. McCain's 4th place in Iowa wasn't read as a "loss", in the bizarre expectations game. But regardless of their finishes, Iowa seems to have not moved the New Hampshire Republican electorate at all.
On the Democratic side, the large pro-Obama bounce is obvious. If it didn't show up in the first day or two post-Iowa, it clearly appears after that. A Clinton lead of some 10-15 points became an Obama lead of some 8-10 points.
Cross-posted at Political Arithmetik.