SO... I've had the same problems as most of you on this thread. Skip down, if you want to know what I did...
My story:
Purchased phone-
First week: battery lasting 3-6 hours, with heavy use, but that was normal, to break in the battery.
Second week: battery lasted ~22-25 hours, with moderate use and once/day charging
Third week (after the eclair update on Sept 17): battery was horrible, lasting ~4-12 hours, with moderate use
Talked online/and on the phone w/Samsung rep- said to go buy a new battery (nope, not going to spend more money). Went to a sprint store, four of the sales ppl didn't even know what an eclair update was (disappointing), referred me to a technical support store.
My solution:
My friend recommended for me to do a factory reset, but I didn't because I didn't want to lose all of my data and pictures/etc.
so i waited to see if the battery would get better. But, I backed up my files onto a computer (plug your phone into a usb port, then mount your device, then mass storage- just drag and dump whatever files you deem necessary). I reset my phone last night, in hopes that maybe it would work better. Someone on the sprint forum also recommended for me to do this. I have had the phone unplugged for 5 hours (usually it would be dying by now), and it's still at 85% after a full charge!!!
SOOooOOooo... go back up your files onto a computer and factory reset your phone, see if this works for you! Home Screen> Menu > Settings > Privacy > Factory Data Reset.
BUT, if you are afraid that this will not work... I will update you in a couple of days, to confirm that this solution has worked, if you don't want to risk doing this and it not working (and having to redo everything on your phone). I will keep you all updated, but it looks like it's working so far.
Let me know if you have any questions!!!