I have been throwing around the idea of the new HammerSchmidt drive system for a fixie and sure that it will work. The HammerSchmidt has just been released by Truvativ as a front derailleur replacement system which utilizing a single ring up front with a system of pawls, worm gears and other doo-hickies to create both a 1:1 drive and a 1:1.6 drive all while maintaining tension on your chain. This in effect would create a 24tooth and a 38tooth ring all in one.
This would allow say a bike polo rider to shift on the fly from an easy ratio to accelerate and manoeuvre to a larger ring for covering distance quickly all with the flick of a shifter. This really though has the potential to offer a segment of the market (commuters, singlespeeders, couriers) the chance to have a tensioned system with two gear ratios. Very Cool. The system requires an ISCG mount (used in chain tensioners for DH bikes) but through e13, a rapidly dominating chain tensioner company, you can purchase an ISCG adapter to fit any bike. The only issue is the requirement of a flanged outer shell on your bb to hole the adapter in place. These bb's could become a hot commodity among the commuter community much like the steel road frames from the 80's have become so popular again for singlespeeders and fixie riders.
Who knows if this will take off but I sure as hell am going to try and get my hands on one. At this point they are only making the HammerSchmidt for the DH, Freeride and All Mountain crowd but if there is enough interest from the commuting community it has the potential to create a whole new drive train option. Imagine an 8 spd internal freehub with a HS up front. 16 speeds and all basically internal without a derailleur in sight. Is this the beginning of the end for the derailleur, quite possibly. Also STurmey Archer has come out with a 3spd fixed hub (I drool). With that setup you could create a spd fixie. That's absolutely insane. Now is there a market for 6 spd fixies, probably not but it sure is cool.