Just confirmed this today... If you're using iOS 4.0 it is close to mandatory that you also Jailbreak if for one simple reason: Background app management.

iOS 4 does not provide a way (that I know of) to manually quit an application, and unfortunately many app developers completely suck. That's what happens when you have such a popular platform, everyone and their mother suddenly becomes a developer.

(Late) Last night my iPhone was at 100% charge as I removed it from a powered speaker dock as I went to bed. This morning, about 8 hours later, it had no charge left. Just enough to show a red sliver on the battery meter as I tried to power it on.

I'm just about 100% certain this was caused by Squeezecast, an application that acts as a Squeexebox, enabling playback of music from Squeezebox server. The app is insanely buggy and I have no idea how it was approved except for complete lack of testing on Apple's part. It must have continued to consume copious resources even after music playback had stopped.

Under normal conditions you'd have to reboot the phone to get this app out of memory. Having a jailbroken phone however one need only have SBSettings installed with its ability to quit all background processes. Definitely a battery saver.