A number of users have reported reduced battery on iPhones and iPod touches after updating to iOS 4.0.1. These reports crop up after virtually every iOS update, and the problem can generally be resolved by running through the first 3 procedures in the 4 Rs:
1. Reboot (hard reset). Hold down the sleep/wake and home buttons simultaneously for roughly 15-20 seconds, until the screen powers off then an Apple logo appears, which signifies a reboot. Some problems may require (oddly) that this procedure is performed twice.
2. Reset all settings. The reset may restore disengaged hardware components and clear corrupt data that prevents proper connectivity. Navigate to Settings > General > Reset > Reset All Settings. Note that this will erase stored messages, passwords and other information.
3. Restore, but not from backup. It appears that bad holdover data from iPhone backups (especially those created for previous iPhone models) can cause various issues. Restoring as a new phone will delete contacts and other data, but may resolve the problem. To do so, connect your iPhone or iPod touch to your computer, click “Restore” in iTunes, then choose “setup as new phone.”
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Failing the above, try these battery saving tips.