I know people have made multi-speed fixie hubs from time to time. Was a company in Seattle converting Bendix hubs for this.

Here is one such example:

"A Bendix 2 speed fixed gear hub. yes, a TWO SPEED FIXED GEAR HUB.

How does it work? Well, it's activated from a friction lever via a cable (included in the auction). Gear changes are instant and silent. The hub allows a downchange of approx 25% which lets you coast up hils in the saddle instead of grinding up in a big gear. It works superbly. Bendix hubs have the reputation of being bulletproof and nigh on indestructable. "


This is from http://cgi.ebay.co.uk/ws/eBayISAPI.d...MEWA%3AIT&rd=1