With the release of iOS4, my iPod Touch (2G) gained the ability to remain connected to Wi-Fi even when locked or in sleep mode to receive push notifications (or VoIP calls in the background). The implications for battery life however are not immediately apparent.
Having lived with iOS4 on my iPod Touch since WWDC, I was initially caught off guard by the significantly reduced battery life. Previously my iPod Touch could go about a week without recharging, as I used it mostly for listening to audio an hour or so a day, and occasionally for looking up other information. After installing iOS4, battery life dropped to a couple days, or even less depending on the application I was in.
It turns out the WWDC application needed to be updated for iOS4 to conserve battery life, but more importantly, it had enabled "Notifications", the 3rd item under Settings. With notifications enabled, an iPod Touch will stay connected to a Wi-Fi network even while in sleep mode, thus burning through the battery much faster than I had previously experienced.
If you use an iPod Touch mostly as a music player and don't normally need notifications, be sure to leave this turned off in the Settings application, or select Airplane mode when you don't want the radios to remain active.
Several users report their battery is still being drained while their iPod Touch 2G is sleeping. I believe this means other applications are activating the radio. Since the iPod Touch 2G doesn't officially get "multi-tasking" even with iOS4, this means one or more of Apple's built-in applications which are authorized to run in the background are implicated. I see two solutions so far:
(1) Use Airplane mode to explicitly prevent the radios from powering on.
(2) Review your iPod Touch Settings and disable any background network access.
Settings -> Notifications = Off
Settings -> Mail, Contacts, Calendars -> Fetch New Data -> Push = Off
Settings -> Mail, Contacts, Calendars -> Calendars -> New Invitation Alerts = Off
Settings -> WiFi -> Ask to Join Networks = Off
I hope Apple adjusts the default settings or provides an option to restrict network access while the iPod Touch is locked so unsuspecting iPod Touch users are not left to resolve battery life issues on their own.