when I was building my fixie, my LBS guy was talking about one that he built up to train his legs for high-cadence efforts. he used a 32T/16T setup, and he said he was cruising at like 110 rpm.

For mine I used a 42/16 and just rode it to work (3 miles each way). I think it helped a bit, but I have a crappy seatpost on it now without enough setback, so my fixie bothers my knees.