My biggest culprits are GPS, wireless, and screen, in that order. GPS (obviously, constantly downloading map bits); wireless -- even when there's no wireless connected, it's constantly taking inventory and trying to connect, so if I'm not at home or at work, I turn it off. Screen -- she's right, you basically mean you don't use your phone. If you turn on an audio app and then screenlock it, I bet it still uses up your battery life.
But then again, I still have an original 1st gen Motorola Droid. I don't want to have to switch to a ceilinged plan, so until I have time to figure out how, I don't want to upgrade.