Woot.com has been putting up the Energizer XP8000 battery. Today is the cheapest price I have seen them post. I picked one up the last time the put it up. If you search the exhibition forum for my motorcycle led project I powered the setup I had in the youtube video with that battery, along with a yellowjacket and fonera router, for about 5 hours. So a arduino only project should last a long time.