It's not fixed, but there's a weird way of making what is basically a two-speed singlespeed: putting two freewheels on the rear hub and routing the chain in an unorthodox manner will give you a high gear ratio when pedaling forwards and a low ratio when pedaling backwards. The bike still coasts normally because of the freewheels, but you can not roll the bike backwards or rotate the pedals while stationary.

There are several pictures of my retro-direct conversion being built up here:
http://basementfreaks.com/gallery/ga...16898772e5dbdc