I don't think turn signals on a bike would be very effective. Most of the time drivers are coming up on you so fast that they wouldn't have time to realize that it's not just a blinkie. Hand signals are more visible for much farther. Anyone who can't figure out that when I'm pointing left I'm going to go left, right for right, probably can't figure out turn signals either.

This is backed up by their own behavior too; hardly any of the cars around here use turn signals, and when they do it's to indicate that they're about halfway through the turn already. I still haven't figured that one out; sometimes you see just the first blink from the turn signal as the car disappears around the corner.