I've seen that happen before, the cause was an e-mail that was stuck in the outbox. So as soon as that e-mail account was added after a restore, the phone never stopped trying to send it. After having removed it from the outbox via the mail service's web client everything was back to normal. No matter how many replacements we would've gotten, it wouldn't have helped sine the problem didn't lie with the phone itself.
I'm not saying it's what happened to you, but it's well worth a look.
If you've got a mac, you can download Xcode (free), then open it, register your phone in the program (connect it and hit a button, no big deal) and open Instruments. In Instruments, you can (amongst other things) look at which processes are active, how much RAM they use, how much CPU etc (an activity monitor). Take a look and see if you notice anything out of the ordinary.
My experience with sudden battery drain is that it's almost always due to a software error that is easily fixable once the cause has been determined, so check it out if you feel like saving yourself a trip to the Apple Store.