Yeah, I could either run 20 LEDs at half power for more visibility, or 10 LEDs at full power for the same brightness. I haven't decided if I will concentrate the LEDs in one place (the end of the rack on my bike) or if I'll make two LED bars of 10-12 LEDs each that will fasten to my drop bars. The LED bar would be encased in some sort of diffusing plastic. If I chose the second option it would certainly be quite visible from the side and back, and would be different enough to catch motorist's eyes. Certainly in either case I would use a series/parallel combination so that I could use as few voltage drop resistors as possible.

I have 7.2V, 5400mAh LiIon battery to work with, but that has to drive both my dual (or perhaps even triple...) 3W luxeon array as well as any tail lights. I'll have to check what the maximum current drain on this battery is, as long as it's 1 amp or more I should be OK. I'd prefer 1.5A, though.