By far the industry standard for PC based OBDII scanning is the EASE corporation:
I am sure they are working on a palm device if they haven't got it already. They have an inexpensive single protocol program for 2-300 last time I looked. They also make some interesting OBDII attachments; including a "black box" that will rat on your kids, employees, wives, husbands etc. It hooks to the OBDII connector and reads out such things (programmable) as top speed, top engine speed, top temperature, times of operation, miles driven, etc. It is wired such that removal is also recorded.