Are there any MMOs targeted on mobile phones (or at least HPCs)? To my mind, this is completely implementable on modern hardware with GPRS and hi-res hi-color video output.

Funny you should mention it, it seems the new mobile console that just came out in Europe called the Gizmondo has something similar to that. Its not really a MMORPG per se, but it uses the built in GPS capability to let you battle with people who also has the device and physically comes into your territory. Its an interesting idea, just not sure how the implementation is.

http://www.gizmondo.com/

Seems the game is called Colors...an urban warfare game.

http://www.gizmondo.com/games/item.asp?id=4&cat=4&all=0

The closest thing I can think of is Pocket Kingdoms for the (ick) N-Gage.