Facing an avalanche of customer complaints about poor battery life in iPhones that are running the 3.1 firmware release, Apple Support has sent a selected group of users a profile enabling a power-logging mode, and has disseminated an 11-item questionnaire, including questions about email use as well as the frequency with which WiFi and Bluetooth are enabled.
The Battery Life Logging mode apparently writes to the user area of the phone's flash, as the test users are instructed not to sync for a few days while gathering data. They are also asked to disable the mode once the testing is complete, presumably to avoid flash overrun due to the growth of the log files.