You probably know by now that the iPhone only lasts about 300-400 charges —and that's most likely full charges, not topping it off from 80% to 100% at night—before you have to send it into Apple for a replacement.[Updated: That's wrong. Here's proof.] Apple's just thrown up a page saying it's going to be $79 with $6.95 shipping, which is $85.95 per phone when you send yours in.
Sending it in also wipes out all your saved data, so you'll have to resync it (but it also wipes out your text messages and email, which you may not be able to resync). Hopefully nobody will have to deal with this for another year or two. Apple Battery Replacement Program [Apple]