Nate Silver highlights them:
[A]lmost any winner of Iowa apart from Mr. Romney himself would present some kind of threat to him in one of the next three states. (The exceptions are probably Michele Bachmann and Mr. Santorum.) Mr. Romney certainly does not need to win Iowa to win the nomination. But unless he builds up more of a cushion in the national polls before the voting there, a loss for his campaign in the caucuses would at least make for an exciting January.
The Dish 