Not talking about the "big butt" extended batteries, but there is usually a larger battery still capable of fitting the regular compartment. Usually there is one bigger one that fits and one SUPER big one that needs a new battery door. Not ruining the look.
For example-
http://store.androidcentral.com/seidio-innocell-1600mah-extended-battery/8A37A6444.htm
The talk/standby times on HTC's website seem in line of most smartphones. Again I see no reason for the 800mhz processor other than to increase battery time, so I don't think they overlooked this problem. Guess we'll see soon.
Talk Time Up to 400 minutes for WCDMA
Up to 590 minutes for GSM
(The above are subject to network and phone usage.)
Standby Time Up to 430 hours for WCDMA
Up to 430 hours for GSM
(The above are subject to network and phone usage.)