I've never seen a battery powered USB keyboard.

From my testing, it would appear that most devices that include their own power supplies still don't work without at least a little bit of power from the USB bus - I've tried a hub with an external power supply, a digital camera, and a wall powered USB CD burner. My 770 was in host mode, with a correct cable, but it failed to work with them.

My theory is that the bus interface circuitry is powered off the USB host no matter what. I'll be doing some more testing to see if that's the case - I haven't wired up an external 5v power supply to the USB bus yet.