One of the most common problems reported with the iPhone 3.1 firmware update is significantly reduced battery life. Apple has noticed the issue, and some AppleCare employees have sent requests for more information from affected users.
Until now, Apple has recommended that 3.1 problems can be solved either with a hard reboot or a complete restore using iTunes. The restore method results in a reset iPhone and the owner has to reconfigure settings from scratch.
Assuming this may not even fix the battery problem, to hard reboot just hold down the home and sleep buttons until the iPhone turns off.
Some iPhone owners have been asked to install diagnostic software that will log application use and power consumption. According to reports, the logging software has been accompanied by a detailed questionnaire to try and narrow down the cause of the problem.It's possible Apple didn't realize some component of iPhone 3.1 software is using the processor more than version 3.0 and as a result drains the battery twice as quickly. Others have speculated that 3.1 firmware somehow distorts the actual battery meter.
The outcry on Apple's forums following the release of iPhone OS 3.1 made it clear this is a significant problem. With any luck, Apple's investigation will track down the root of the problem and users will soon be able to update to 3.0.1.