I just did this and it works ten times better. My battery was dying in a few hours without me even using it. The battery replacement was quick and the phone works like new. below is a link to the guide and the parts. iPhone 3G Replacement Battery
I just did this. If you hold the screen at 45, while using the plastic spudger to hold the ribbon down, you can push with the screen back into the slot. Brace the bottom of the iphone with the heel of the palm that is holding the plastic spudger to steady the base as you slide it in.