I have been using an iPhone 4 4.0 jail broken and UltraSn0w unlocked phone for several months now. I am on AT&T but travel internationally and use a prepaid when I travel. Unlock was done with jailbreak me.com
Last week I started seeing miserable battery life, like maybe 5 hours on standby from a full charge.
After uninstalling most everything, including the unlock with no change, yesterday is restored to 4.0.1 and then jail broke again but didn't install anything else other than the cydia essential updates. Charged the battery and then let it sit over night. Totally dead this morning
Restored it again to 4.0.1 this morning and fully charged it again. Been now about 6 hours and the battery gauge is still almost fully charged.
Any ideas what could be causing this?

