GPS receivers are getting smaller and smaller: this one fits on your wrist. Okay, so it won't give you turn-by-turn directions, but the Casio GPR-100 GPS watch will keep track of how fast and far you've traveled when you're on a run or otherwise zipping around the place. And, as it uses the GPS signal, it should be accurate to within a few thousandths of a second. The Google translation version of the specifications of the watch are here.
Casio GPR-100 Watch [MobileWhack]