I just picked up a Seidio "OEM-size" extended battery for my HTC S620 (unlocked UK version of T-Mobile Dash). It's 1300 mAh instead of the 960 mAh stock battery. I've gotten a nice bump up in battery life even with heavy web browsing with both cellular and wifi connections, making calls, and playing a Sega Genesis emulator. They make similar stock-sized batteries for most of the more popular smrtphones. Might be worth a look if you need some more juice but don't want to keep on a charger or have a big bump sticking out the back of the phone.

www.seidio.com