Battery dead in 3G and dying in 4G
The 4G repair went "as smooth as butta". The 3G was a bit more demanding.
On the 3G, when I lifted the display slightly, the wires labeled "3" were already disconnected. After replacing the battery and putting everything back, the display no longer worked. The battery works because flipping the Mute switch on the side causes a short vibration. I'm guessing that I broke a connector when lifting the display. Be VERY careful.
iPhone 3G Replacement Battery
iPhone 4 Replacement Battery
iPhone 4/4S Liberation Kit