According to Apple's release notes, iPhone OS 2.1 delivers "significantly better battery life for most users." "Most" appears to be the operative word in that sentence, as a number of users are reporting seriously degraded battery life after the update.
One iPhone Atlas reader, Jerry Flaby, writes:
"Since updating to 2.1 I have seen a dramatic decrease in battery life. I can make it about half a day with moderate use as opposed to a day and a half before the update. My usage meter continues to grow when I am not using the phone. I have tried soft reboot, hard reboot, restore from backup, and reseting all settings. None worked!! Some people have narrowed this down to a push issue with both MobileMe and Exchange. I personally have used push with exchange with no problems, until now. I have turned off push and will see if it helps. This is by far the worst problem I have had with this phone. It makes text lag and reception issues look like a joke in comparison. Also, I cannot get a fix with GPS while 3G enabled. Works fine with 3G disabled."
Flaby's sentiment is echoed across Apple's Discussion boards.
Among the workarounds and fixes that have proven at least semi-successful with regard to boosting battery life under iPhone OS 2.1 so far:
Drain the battery completely Use your iPhone until the battery dies completely (indicated by a battery icon appearing on the screen when the sleep/wake button is pressed), then fully recharge the device. It appears that the full discharge may reset circuitry that prevents a full charge from occurring, despite iconic indication of a full charge.
Turn off location services It appears that the Maps application and some third-party applications that use GPS functionality fail to power down the necessary hardware components when they are no longer in use. Navigate to Settings > General > Location Services and turn these services off when you are not using Maps or another location-enabled app.
Turn off Push then restart The iPhone 3G?s Push functionality for Exchange and Mobile Me accounts can destroy battery life. Turn this option off in "Settings" then restart your iPhone by powering it off then on.
Restore the iPhone with original settings Launch iTunes and select the second option in the "Version" section on the Summary page for your iPhone. This restore process will erase your iPhone software and data, including all settings, which can be restored later via iTunes.