I went through this as well...back and forth with Apple regarding my brand new MBP. I ended up *very* annoyed (they had me send in my computer, but didn't end up doing anything to it!) because this is how they determine battery life (which is, of course, completely unrealistic...no one uses their computer this way!):
1. Fully charge the computer.
2. Set the display brightness to 50%.
3. Turn on AirPort.
4. Turn off Bluetooth.
5. Remove any external devices.
6. Disable sleep and screen saver.
7. Disable any background processes such as Time Machine backup or third-party background applications, file sharing, internet sharing, and so on.
8. When you've completed these steps, remove AC power, start a timer, and let the computer sit with no applications open. Be sure to let the computer run until it forces itself to sleep without any interruptions.
9. Once it is asleep stop the timer and write down the hour and minutes the computer stayed on before going to sleep.