Emily's new iPhone 4S can't make it through a day of light use.
I suspect a software issue with either IMAP or MobileMe or ActiveSync polling. Otherwise, hardware. I posted a question to Apple Discussion: iPhone 4S battery life is half of what to solicit ideas.
I'll see if a software notice appears in the next few days, and I'll post updates here.
Update 10/16/11: This post has a likely cause. It's apparently a longstanding iOS bug. Restored accounts may have problems with IMAP/POP accounts resulting in high server polling rates. The fix is to delete and restore the account. I did that on Emily's account, and on my first try with the restored account Mail.app crashed. On the 2nd try I had to reenter her password but it's working now. She is getting new email, but I didn't notice a problem before.