I know there are people in the Geekiness thread of Commuting that know a lot about lighting, but I mention it here because it also has a lot to do with safety. (And probably some of you here also hang out there.)
I've been thinking recently about the feasibility of having two side-by-side red rear-facing lights that could function as a turn signal and brake indicator for a bike, just like they do for a car. This setup could be mounted on the seatpost, rear rack, or perhaps a wearable vest. The turn signals would of course need an activator switch of some sort near the handlebars, and perhaps there isn't a good way to get them to turn off automatically after the turn, so you might have to do that yourself. That might be a pain. As for the brake lights, there ought to be some way of integrating the wiring with the brakes so that gripping the brakes would close a contact and turn on both lights as once to indicate stopping.
The only drawback I can think of is if the lights would be large and bright enough to really be noticeable in car traffic, since they need to be battery or generator powered. Even so, the brake lights would also be useful on group rides, where they would be more noticeable when only surrounded by other bikes.
A quick Google searched turned up one good lead on a turn signal, but nothing on brake lights except for a Chinese portal site for bicycles which didn't seem to have anything about lights when I went there. Maybe a more thorough search would turn up more.