hey
try this, it might work for your
it worked for me !!!!!!
i had the same problem with my iphone replacement 3GS with 4.0.2 iOS
i took it home, activated and let it charged until full.
then i unplug it and 7 hours later i checked it, it was 55% battery life left
i didnt do anything with it, no even call or surf on the net
it doesnt even have the sim card on (only stanby)
so looking at the setting
i saw the internet tethering spinning and spinning never stop
i didnt want to restore it as it has 4.0.2, dont want 4.1 yet
so i put my sim card
turned the 3G setting off
turned the celullar data off
internet tethering still spinning
then i turned the 3G setting on (internet tethering still spinning)
turned on the cellular data on (internet tethering still spinning)
turned on the data roaming on ( voila!!! internet tethering OFF )
then turned data roaming off and cellular data off
and internet tethering still off
let me know if works for you
i will compare the battery life if this was affecting or maybe something else