I've never actually come across mobile paperball games before, but this one looks pretty good. Using the accelerometer onboard every WP7 device, 3D paperball let's you indulge in that traditional office pastime while saving some trees in the process. The game is ad-supported so you don't pay a dime to try it out.
"A planner is a gentle man, with neither sword nor pistol.
He walks along most daintily, because his balls are crystal."