I was one of 2 riders this summer who did the Seattle Tour de Cure century on a fixed gear. 101 miles, around 6500' elevation gain. You can watch the gradual degredation of my physical state in video by searching out my username (same as here) on Qik.

I will likely do other century rides and maybe even some 200k brevets on the singlespeed side, but certainly not fixed. Totally kicked my arse harder than any 300k or double I've ridden.