Could this be the perfect festival bag?
Pacific Outdoor Equipment e-anchorage
Appears to charge devices via USB and 12v car adapter interfaces. Couldn't find specifics on output and have inquired from the manufacturer. Will post when/if I hear back. Also available in a single solar panel model the e-vancouver.
For more info
http://www.pacoutdoor.com/2005/index.cfm?action=product&productID=120&groupID=13&familyID=2<edit Sept 02, 2006 After three emails to Pacific Outdoor Equipment's customer service contact as listed on their website, I have received no response to my request for output specifications of the e-Anchorage and e-Vancouver bags. It may, or may not, be an ideal bag for taping purposes but based on the no response and the hefty price tag of the two models, I won't be spending any money with Pacific Outdoor Equipment for these bags or for anything else they sell. /edit>