I had a chance to ride a Retrovelo Otto Duo while I was in Portland. It had one of those cool Schlumpf planetary cranksets and it was awesome! I asked the Retrovelo guy if the Schlumpf could be used on a fixed gear and he said they were working on a version that could handle fixed gear riding. They make Unicycle hubs that shift so I'm guessing a two speed fixed gear crankset would be reasonable. If the ASC fixed three speed ever happens you could have a six speed fixed gear touring bike! He also said that he thought the current version would handle fixed gear if you weren't skidding and stuff.