http://www.ladyada.net/make/mintyboost/
I have one myself, I'm currently using a Tenergy 6600mAh 1S LiIon with a built in PCB board..
Thing rocks!
I can get a good 4-5 charges on my iPod, and 1-2 on my phone on a single charge!
Instead of the two double AA's, I soldered up a JST connection, and hooked the battery into that connection.
I'll grab some pictures afterschool today..