I use the Socket Power Pack. That thing is awesome. Sure, it's bigger than the Axim, and weighs about three times as much, but it just keeps going... and going... and going...
I took it on a trip, ran my iPod nano off it for a total of about 24 hours, came back, plugged my Axim in, and it ran that for another 9-10 hours. That's with wi-fi, bt on, and playing music.
Of course, I also have an external laptop battery with a USB power out port... about 5 times bigger than the Power Pack
