I use my HTC Incredible. I put it inside my saddle bag, my jersey or my tri short's side pockets. Currently use the MyTracks app and it's been pretty accurate. Except on a short, extremely steep incline ride (has a part where the grade is 23%) that lasted a little over a minute (it recorded that I went 31mph avg speed and over 1000 feet in elevation; I wish). My buddy's Garmin went psycho on that ride too, so... meh.

I also use my Cateye Double Wireless to double check the figures on the MyTracks and to keep an eye on cadence + current mph.