Charles Stewart III has a really nice quickie analysis comparing the county-level Republican caucus votes in Iowa in 2012 and 2008. There are a couple of cool findings in there. One is that the strongest correlation from one cycle to the next comes from the Huckabee '08 vote versus the Bachmann + Perry + Santorum vote -- basically, the social conservatives. It reinforces the idea of the Iowa Republican caucus goers being divided pretty neatly into three persistent camps: social conservatives, business Republicans, and libertarians.
The other cool finding was that Romney's 2012 caucus vote correlates slightly better with McCain's 2008 vote than it does with Romney's 2008 vote. This means... something. I'm not entirely sure what. I like to think that it means that Romney, the likely nominee this year, absorbed some of the infrastructure and tactics of McCain, the eventual nominee four years ago. But that may be a stretch.