PDA problems are
a) screens
The not-cheap polarised screen, variously called "all-view" etc etc and available on tablets, doesn't appear to be out on PDAs yet
b) accidental touching
It is so easy to b*gger up a PDA GPS by touching the screen. I thought about getting a pocket/pc expert friend of mine to write a little driver which (when executed) would capture and dump all keystrokes but in the end I never bothered. Obviously one has to disable all the preprogrammed buttons, but not all buttons can be disabled, IME.
Whereas a tablet PC comes with an inductive screen which can be touched without problems.