I've got one of these:

http://www.premiumpowerproducts.com/.../PowerBank.htm

and I've been happy with it. Charges from AC or via USB (laptop, no drivers), comes with tips for a wide variety of devices (iPod, Motorola, Treo, mini-USB, standard USB, etc). Most importantly for me, it is iPhone 3G compatible, and has enough power for about 2-3 full charges of the phone.

The one brick-and-mortar place I've seen it is Fry's.