PocketMatrix

I use an XDA, and would certainly be interested in a good online person-person game. One thing to note is that GPRS users (at least here in the UK) pay by data transmitted, so keeping transmitted information to a minimum would be useful.
I'd agree about keeping it turn-based as the PPCs internet connectivity doesn't seem capable of anything real-time.
I haven't played any existing ones, but would be interested to hear if anybody else has.

Snails is awesome fun over WiFi - if you can find a game.