When I was walking around with the BeagleStache [4], I was using the
BeagleBone Battery Cape [1] that Gerald mentioned.
For my talk, I used an Anker USB battery [2] and IOGEAR
Ethernet-to-WiFi bridget [3]. That battery provided longer term power
and an additional port to power the WiFi bridge.
[1] http://circuitco.com/support/index.php?title=BeagleBone_Battery
[2] http://www.amazon.com/Anker®-10000mAh-External-Battery-Android/dp/B005NGKR54
[3] http://www.amazon.com/gp/product/B004UAKCS6/ref=oh_details_o04_s00_i01
[4] http://beagleboard.org/project/stache