Well the TMobile G1's extended batter came out 2-3 months after the phone came out...
Seidio released a larger battery with a new back cover because of the size of the battery and that is the one I got. I doubt I will be getting an extended battery for this one unless the physical dimensions of the battery are the same.
This phone is SO thin and I would not want anything to change that.
At least from my experience, it isn't a clear double, but it is a 90% increase, which isn't far off. When I owned the enV, and obviously less battery hungry phone, but it was significant enough of an increase I'd only need to charge it roughly every week... 8 hours of talk time is a LOT... Ialso found out that the bluetooth radio draw is significant because even though my Bluetooth denecessitated the need for the speaker to run, I'd only have a six hour battery life of talking. It was enough battery I'd forget to charge it because I wouldn't watch to charge it. I have the plastic blue hard case on my Motorola Droid that they sold at the Verizon store and it works REALLY well and I'd really not like to loose it. I wish they'd realize there's a market for cases with eztended battery dimensions and such... the stretchy silicone cases worked pretty good though for my enV with the extended battery.
I'm waiting for sedio to come out with these accessories, they have extended batteries that are in the same dimensions as the phone for other devices so I'm hoping they do the same for the droid. Sedio seems to cover the majority of accessories for all my phones and the quality is par none, so I'm waiting on them to release their droid accessories