Actually, I'm already working on a similar project but with different goals. The goals, in order of importance, of my touchscreen are:
1) Be a customized keyboard for PC Games that need the keyboard. By "customized", I mean take out all the "admin" keys that would let the user get into things they shouldn't.
2) Show controls.dat info for MAME games while you're in playing a game (for reference).
3) Show high scores for MAME games.
4) Instant replays for MAME games.
#1 is actually the easiest (fortunately), and is completed using a custom network application that I developed. The rest are pending

My "touchscreen" is actually a 3com Audrey and since it runs QNX (a flavor of UNIX), it is very limiting to me since I'm having trouble getting my hands on an SDK. If I have a Windows based webtop, I'd be able to make 2-4 work

Until then, I can only give you what I got.
-sab