The source is free for any non-commercial use (GPL), so as long as you arent planning to sell it its free to use.
PocketPC Port homepage
Quake'ish FPS demo from this page, works on smartphone: DEMO
I tried to run the DEMO on my mpx200 and it WORKS! Although it only uses about 2/3 of the screen, I get +/- 20 fps, i can walk around, jump and shoot, and the music and sound works. The graphics quality is comparable to Quake but frame rate is about 4 times as high.
See this for yourselves! Note that when you start the .exe, you'll see the credits for about 1 minute before loading is done and you can play. Walk around with the d-pad, shoot with left function key, jump with right function key, enter menu with back-key. Quit by going all the way down the menu and hit the left function key (shoot).