I'm completely sold on 18650 LiIon cells for flashlights. The difference between those ans AA cells is astounding, and they're less trouble than CR123A rechargables. Whatever you do, you're going to want rechargables anyway.

I agree on the MagicShine thing - the battery can go absolutely anywhere, it'll strap on any tube on the bike or go in your pocket with an extension cord (though I'd only use that if I helmet mounted).