I had the problem with eternally rebooting iphone 3gs after replacing the battery with a non original replacement battery. reboot every minute or so, no update on battery loading etc.
Opened the phone and the label said "Battery for Iphone 3G". It was raining and I didn´t want to go to the store and I thought f*****g drm issue. But probably solvable with a soldering iron.
I disassembled the original battery with some caution. Then I measured what was + and - on the tabs spot welded to the PCB. Marked out was was + and - and did the same for the replacement battery. Then I transferred the PCB from the original battery and attached it to the new. Problem solved! no rebooting. now i have a 1600mah battery in my 3gs, wonder if it will work better than the original, Exciting but nothing has exploded yet ;)