I recently decided to violate my “never install a dot zero release” rule and installed iOS 4.0 on my iPod touch. The features that I could use on my second generation hardware were nice, but I immediately noticed that the battery drained in a couple of days, even though the iPod was in sleep mode.
Apparently there is a bug where the Wi-Fi will stay on even when the iPod is asleep. That does no good because none of the applications that could benefit, like email, stay on.
I’ve found that the workaround which does the best job of preserving the battery’s charge is to switch the iPod into Airplane Mode. The switch to turn on Airplane Mode is right at the top of the Settings screen.
In practice, I turn on Airplane mode and then hit sleep. When I unsleep, the Settings screen is still there and I just turn off Airplane mode. It is a couple of extra steps, but it is doing the job for now. I had high hopes for a fix in iOS 4.0.2, but no luck there. Hopefully iOS 4.1 will have a fix!