Like everything in mobile design right now, the answer is never simple (and notice how these sorts of questions make great research projects).
Here are a couple of academic writings that address the issue (both of which should be available via the WSUV Library):
Target size study for one-handed thumb use on small touchscreen devices, by Pekka Parhi University of Oulu, Finland, Amy K. Karlson, University of Maryland, College Park, MD, Benjamin B. Bederson University of Maryland, College Park, MD
Designing Gestural Interfaces by Dan Saffer
The recommendation from the Apple development camp is: 44X44 px
I've read elsewhere that 60 px diagonally is a good baseline measurement, but the consensus seems to be that the best approach is to create the buttons and, surprise, test them on users. ...