A number of people with iPod Touch devices have reported that their battery life has significantly lessened after installing iOS 4. The problem has been noticed after people left their devices charged in the evening only to find that overnight the battery had reduced. Instead of giving them the expected 4-5 days of use on a single charge with intermittent use, they are getting only about a day's worth of use.
Though some might expect small fluctuations in battery usage with new operating-system software, since the hardware will be handled slightly differently, this should only account for a small percentage difference and not a nearly 80 percent drop in expected run time under the same conditions.
The issue at hand appears to be a problem with Apple's networking services, such that if the device is left on it will continually synchronize and send push notifications for mail and contacts, resulting in high Wi-Fi usage and thereby draining the battery. For some reason, the iPod will not stay in sleep or standby modes with these services enabled, so when put away for the night, the device will continue to use Wi-Fi and drain the battery.
Currently this appears to be a bug in the OS, and unfortunately Apple has not been forthcoming with a fix for this issue, so if you are having problems we encourage you to send Apple feedback on it at the following page (be kind, they're human): http://www.apple.com/feedback/ipodtouch.html.
One user and MacFixIt reader has outlined how this issue has affected a number of people (around 260 so far), and put the responses in a Google Spreadsheet that you might link to when giving Apple feedback.
While we wait for a fix to this issue, there are some ways you can get around this problem:
Turn off Wi-Fi and Bluetooth
When you are not using the iPod, turn off Wi-Fi and Bluetooth in the settings to ensure no wireless data can be transmitted. This has been the most common way for people to avoid the issue.
Use AirPlane Mode
Apple has a relatively new "Airplane" mode for the iOS that prevents Wi-Fi, Bluetooth, cell signals, and GPS from being used when on airplanes so you do not have to turn off your device on the flight. Enabling this on the iPod Touch will have a similar but more thorough effect than manually shutting off Wi-Fi and Bluetooth.
Turn off "Push" notifications
In the Mail and Contacts settings, turn off "Push" notifications and set the device to manually check for messages. This can be done in the "Notifications" settings in the iPod.
Turn off location services
iPod Touch and iPhone devices running iOS 4 will automatically look up Wi-Fi hot-spot information to get a bead on your location and provide you with information in Google Maps, when you take pictures with the camera, and other enhancements. Turn these off if you do not use them, since this will have the device regularly polling the wireless signal sources for location information.
Reset and restore the iPod
Finally, you may wish to try fully restoring your iPod Touch by backing it up in iTunes, resetting it to factory settings, and then restoring the backup. The instructions for how to do this can be found in this Apple Knowledgebase document.