The Beagle Board looks quite nice as a low power device. I'm planning to purchase one to try out some different experiments on, but I'm waiting until there are more tutorials available on how to get it up and running.
From what I've read, the Beagle Board can run on USB power, which is pretty amazing. It can also be powered by a power adapter, which will likely use more power in the voltage transformation from 120V to 5V.
The amazing thing about the Beagle Board is how small it is, how little power it uses, and yet it contains the same level of graphics power as many desktop computers. I'm looking forward to trying one out!