Just replaced an Iphone 3gs Battery.
Disassembling is easy, the problem is reassembling.
Take your time on connecting the cables. When they connect, they connect. It's hard to connect them but once you do it right you'll feel it. I did very light tappings around the connector to see if any part was loose to check if it was all correct.
On the 3 cables from the logic board, connect number 3 first, it doesn't go all the way like it was before, that's ok AFAIK. After number 3, connect number 1 and then number 2. It worked better for me that way.
The battery even comes with a 20% charge, hehe.
Good luck to you all and have a lot of patience. I did it in one hour aproximately.