Here is an interesting article on the unexpected onside kick from AdvancedNFLStats.com:

Advanced NFL Stats: Onside Kicks

Basically the article says that early in games, when the outcome is still undecided and an onside kick is not expected, it is recovered by the kicking team 60% of the time.

He stated that he was going to perform a modified analysis for late-game situations in the next few days.

The author, Brian Burke, has a spreadsheet that lists every play from every team in the NFL from 2000 - 2008, and he combs through the data looking for interesting trends. If you are a "stats guy," he really takes things to the next level. The math is not complicated if you can keep track of the high number of variables. He discusses 4th down playcalling (go/punt/FG), run-pass balance, home field advantage, etc. It will make your Friday go a lot faster.