I'm not sure if this is the correct forum for this topic but I wasn't sure where else to put it, so here goes! I'm not sure if anybody ever posted about this before.
A new "twist" on the single speed craze is the two-speed transmission, where a conventional double crank and front derailler is used, but the rear gear is a single cog.
This setup is a little more flexible than a SS for commuting in a hilly area and for club rides and light touring. It would also probably be ideal for MTBing. You would still have the advantage of a dishless rear wheel (and SS simplicity) with maybe one more, perhaps low, gear to get over a tough hill. This would be an ideal setup for a frame with vertical dropouts so a Surly Singlator (or equivelent) could be used. I've done this to my early '80s Peugeot road bike and it works great!
My local shop specializes in single-speed and fixed gear and they are doing these conversions.