Xpilot does use UDP for game data. The reason you suffer lag and bad modem performance is that Xpilot doesn't use client-side prediction like Quake.
Unfortunately, this means Xpilot is pretty much unplayable on a modem. However, if you have DSL, cable, or a fast work/school link, then Xpilot is a fantastic (addictive) game.
I suggest people check it out, because the game needs new blood...
An open, cross-platform journaling program.
A scientific plotting package.