I ran across this recently: http://www.usmodular.com

The PowerAid (dumb name) has a capacity of 3400mAh and it's only 13mm thick (according to the web page, or 6.5mm if you're looking at the pdf ).
Froogle shows a bunch of companies selling 'em for around $31.
It's not obvious whether the USB cable is for recharging it or for powering devices with USB sockets (their marketing dept is obviously not very clued).
The main problem is, it doesn't seem to come with the mini Nokia tip, and I don't know if they offer additional tips (again, *****s in marketing).