Well, it's finally happened: we have our first poll showing Mike Huckabee overtaking Mitt Romney for the lead in Iowa. Rasmussen has the goods (Nov 26-27, 839 GOP LV, MoE +/- 3.5%):

Huckabee 28 (+12 vs. last poll Nov 12)
Romney 25 (-4)
Giuliani 12 (-3)
Thompson 11 (-3)
Paul 5 (+1)
McCain 4 (-2)

Overall, Romney still leads by 2.8% in the RCP Average for Iowa, but the trend lines are fairly ominous for him:

Meanwhile, Huckabee has also passed Romney on Intrade as the favorite to win Iowa. Huckabee contracts are now trading at 49.0 while Romney's are at 45.4.