updated 04:34 pm EST, Fri February 8, 2013
May be traceable to Exchange glitch
[Updated with info from AOL on the "meeting managment" bug] Some iPhone owners are complaining about battery life and/or overheating after upgrading to iOS 6.1, according to multiple threads on Apple's support forums. The exact cause of the problems is unknown, but one owner notes that their phone appears to have been running something in the background, and that a factory reset fixed the situation. Some reports specifically suggest that the problem is related to Mail's handling of Exchange.
iOS 6.1 is believed to be creating corrupt meetings in Exchange, which can force devices into a sync loop and play havoc with Exchange servers. AOL has sent a memo to its iPhone users noting that the meetings bug causes "unusually high traffic" and requiring its employees to temporarily disable the meeting management ability on iOS devices using 6.1. Excessive sync attempts could explain unexpected background activity, and especially power drain, since constant wireless use will rapidly deplete mobile devices. If the Exchange issues are indeed the root of the problem, it would also explain why many iPhone upgraders are not experiencing the bug.
AOL said it is working with Microsoft and Apple to resolve the problem. A temporary fix for Exchange trouble can be found on DevCentral. Otherwise, people may have to wait and see if iOS 6.1.1 solves the glitches.