I think the only real way to test this is to put the phone on airplane mode with the screen 100% bright with no autolock and see how far down the % has gone in say 2 hours, then upgrade to 3.1 and repeat. Too many factors to test it logically any other way. However, after a few days I have noticed an increased battery life with 3GS, not sure if it's 3.1 or because battery has been charged and discharged quite a few times since 3GS was released and the honeymoon is over.