The headlight assembly (with two lights) is labeled "LP-M300" -- "Xenon" on one side and "High Brightness LED" on the other. It has two bulbs, both powered by the front hub generator: one is an LED bulb which is good for people seeing your bike. The other is a 6 volt, 2.4 watt Xenon bulb, which is very good at illuminating the path in front of your bike.
Well, it is until the bulb burns out, which happened a few months ago, after i'd been riding the bike about 5 days a week for the previous year or so. I tried the bulbs that REI had in stock, but they didn't last longer than a 15 minutes. I googled for several hours on several occasions, but the best thing i could find, the Union LT5004, (also 6v, but 4w) would NOT work -- the glass globe was too large to fit in the light housing. So don't order that. :) I sent email to REI's online customer support, and didn't get any useful help there.
After riding around in the dark for awhile, i broke down, googled contact info for Shimano, and called them directly. In about 5 minutes (guh!) i had a part number that my local REI could order for me: Y76Z17000. I had to order at least six, but i don't mind -- they were $4, which is about average for bulbs like this, and i'll probably eventually use them. The bulb package also says on it: LP-N010 Electric Bulb (6v - 2.4w) (Xenon).
Hopefully this info will eventually help someone else. :)