I gave mine another full charge from dead last night, all apps killed and rebooted.
Unplugged it at 11.55pm at 100%, got up this morning and at 8.25am it was at 90%! I checked the usage and it recorded 8 hours 30 mins of standby and 24 minutes of usage, weird huh?
What I want to know is why the phone was using itself for 24 minutes in the night?
An hour later it's on 82% and all I've done is check an email, a 1 minute call and go on the app store very briefly. There's another bunch of awdd messages in the log too.
Debating if I should DFU mine or not, but since it was set up as new I'm inclined to wait.
Trying to decide what to do ... to return or no to return, the 14 day cooling off period is almost up! There's no apple store near me so a genius appointment is not an option. :-(
For those who don't want to set up as new because you'll loose app data (lets say angry birds/cut the rope) you can extract the data from a backup using "iPhone Backup Extractor" and place them on the phone using "DiskAid" (Google them) both have free trials that would do the job for you and does not require jailbreak.