Recently I got a few emails complaining about the battery drain issue after iOS 4.3.5 upgrade. Personally I also experienced the issue a couple days ago.
My wife accidentally upgraded the iOS while syncing her iPhone with iTunes. That’s fine as she doesn’t need jailbreaking. However, after the upgrade, the battery drained so quickly that she needed to recharge it after half-day use. It’s just normal usage – a few phone calls, couple email checks and some web browsing. The battery life even decreased quite significantly while the iPhone is just in sleep mode.