A number of iPhone users are reporting battery drain and overheating issues possibly related to the newest update From Apple Insider:

While the extent of the 3G problems Vodafone reported is unknown, multiple Apple Support Community threads exist regarding battery drain and overheating issues, which appear to be affecting a number of iPhone models.

The Support Community forum battery drain and overheating problems after updating to iOS 6.1. Reports are conflicting, however, as one iPhone 5 owner saw a drain of 35 percent overnight, while an iPhone 3GS user said Apple's latest update actually boosted battery life.

 Apple's iOS 6.1 reportedly causing 3G and battery issues for some iPhone users