I've toyed with the idea of trying to rig up a system for a fixed gear anti-theft system. Mount a rear brake (or if you ride brakeless, front brake) lever on your bars, and have it activate a canister of pepper spray contained inside the bars. Use brake cable to direct the spray at where the thief's face will be when he's riding.

Of course, I haven't worked out the mechanical details of the system since ya know, I'm never actually going to do it.