From Androidforums:
Press HOME, and bring up the dialer. Dial *#*#4636#*#* (Do not press "Call"). After a second, it should bring up the "Testing" menu.
Go into Phone Information, press MENU, and tap "select radio band." You should experience a Force Close, don't panic, this was supposed to happen. Scrolling down shows that the preferred network type is GSM auto (prl).
What this means is that even though you are connected to a CDMA network, your phone is trying to find a GSM network, preventing the radio from sleeping, and causing excessive battery drain.
Remedy this by tapping GSM auto (prl), and changing it to CDMA auto (prl).
Has increased battery life for me by alot.