wow... I did quite a bit of research on them about a year and a half ago.
What I found out is that GM, Ford, Chrysler and the Japanese cars all use slightly different 'add ons' to the OBD protocol.
Sooooo
You could get a single purpose scanner for like $200 (PC or Palm) or you could splurge and get the multi-vehicle version for $400.
At about that time some garages in the area started offering up "Free Computer Diagnostic Hookups" so I killed the search and lost all the links I used to have for it.
IMHO Getting a PC version would be better if you have access to a portable PC. The hardware for a PC (even a old slow one) is better than a Palm device and thus could allow more functionality, easier to read data layouts etc.
I would imagine that the prices on these devices have dropped over the past 18 months and am interested to see what people find!