A possible alternative to the bulging extended batteries is an external.
I recently picked up an eneloop rechargeable booster with a capacity of 5000mAh and up to 1A output, so it can run the Axim while it is recharging, as though it was plugged into your computer.
http://cgi.ebay.com.sg/SANYO-eneloop...81662340531705
I use it with a usb cable with the booster in my pocket when I am running the gps while geocaching. Its pretty small and unobtrusive, gives me 6 times the regular battery life, and I can use it with my sweet Innopocket Magnesium case.
It was about the same price as a Mugen 2800, though with almost double the capacity and no huge pregnant bulge on my Ax!