I just received an Extended 2200 mAh Battery for my T-Mobile G1. I have not ran any special tests but the original battery was a 1150 mAh one so do the math, 1.91 more battery life. And this is one of the good things that I love from my G1,  I can change the battery freely no like another very popular phone that you all know.

Here are a few pictures of this battery.