Dan Saffer has made some nice illustrations of "activity zones" on touchscreen phones and tablets, based on which areas are easiest to reach given normal ways of holding the devices. Put frequent actions in the "easy" zones and infrequent or dangerous ones in the "reach" zones.
See his post for the rest of the drawings: Activity Zones for Touchscreen Tablets and Phones (Kicker Studio).
For some research that validates this idea of zones, I'd recommend the publications from Amy Karlson of Microsoft Research. She has done a lot of research on one-handed thumb use of mobile phones in particular.