Something I've heard before, and its mentioned in anlther thread, a big contributer to good or bad battery life (exagerated with 3G) is environment, too cold or hot, and more importantly signal strength, bad signal and the phone could ramp up power to stay connected and 3G isnt as good for coverage in many areas either, which would amplify the problem further!
Perversely, someone also told me if you are too close to a transmitter you have too good a signal and that can cause problems too - though I havent seen that confirmed or repeated.
Both of the above can also apply to Wifi so our network expert says...
What i'm getting at is people seeing different usage times isnt a big shock, and its not that one is doing it wrong necissarily.