Several people have noted that a large amount of early votes were cast when Mitt held a 16 point lead over McCain. Not sure how accurate that is, or how significant, but there is a story out of Iowa that basically has gone unreported: The effectiveness of Mitt's ground ops in Iowa.

Remember Mitt's percentage in the final results basically echoed the numbers in the pre-caucus polling, despite the fact that in pre-caucus polling Evangelicals made up about 40% of the sample whereas in the election they made up 60%.
It would be fair to assume that the extra turnout of Evangelicals went almost solely to Huckabee, especially in context of the pastoral network and Ed Rollins comments here. This would actually mean that Romney would have to substantially outperform the pre-caucus polling to even equal the pre-caucus polling.

It seems Mitt's ground opps in Iowa was worth 7-10% but as heavily overshadowed by the amazing Evangelical turnout that consisted of 77% of Huckabee's votes.