as of 4/5/2009 I've used an ENERGIZER ER-DVD-max that is really for extending the runtime for Portable DVD Players.
It's output is 9.6v @ 4500mAh and will run my Chumby that has about 46 widgets running for 14-16 hours.
There has been no difference visually or signal-strength running it on 9.6v.  Might even extend the overall life of this baby!
I've got the brightness at about 30% which is still quite sufficient for all the various kinds of widgets running.
The battery doubles as a stand for the Chumby.
After the battery has been fully depleted, it takes about 8-10 hours to fully charge. (i.e. overnight)
The neat thing about this pack is that it has a build-in battery fuel gauge that shows you how much charge is left, comes with rubber feet as not to scratch any surface and unfortunately does not come with the correct power plug that fits the Chumby but you can pick one up at Radioshack for about $5 (its a type M).
There is one you may find will sort of work that comes with the battery-pack, but if you move the plug around you will soon find the power get interrupted and the Chumby reboots.
Spend the $5 and do it right.