The other day I pulled my iphone out and nearly dropped it! It was burning hot and there was a big red low battery indicator in the middle of the screen! I had charged it only a couple of hours earlier so it was not likely that normal use had caused this condition.
I was in a wireless area, and had been checking my gmail earlier. The little activity indicator at top left was twirling.
A search through Apple support and discussions led me to this article. It seems that the iphone can get stuck in a checking-mail loop, particularly on certain IMAP accounts like gMail.
An Apple support article also says that a flaw in the Mail application in v. 1.1.3 causes it to persistently generate server activity even when it is not checking mail per user settings. Persistent mail checking can quickly drain the battery.
From the article:
"One fix that has proven especially effective for users experiencing this issue after the 1.1.3 update: Force-quit the Mail application every time you are done using it. This can be accomplished by holding down the Home button for several seconds while in the Mail application until the home screen reappears."
They go on to suggest you delete and reestablish email accounts if the problems persist. A more complete set of fixes is located in this article. Fixes and suggestions covered include: