Reports are coming in of iPhone 4s overheating and experiencing severe battery problems after updating to iOS 4.3.1.
Some users started having problems with the iOS 4.3 update, and it hasn't been resolved with the 4.3.1 update. The problem has been resolved for a few users by removing stale sync threads in Mail or by tracking down other apps, including Ping, which are sending or receiving large quantities of data while the iPhone is in standby.
Apple issued a warning about iPhones overheating due to external temperatures back in 2009, although this time no users report seeing the famous "iPhone needs to cool down..." image yet.
Are you having overheating and/or battery problems with your iPhone? Have you managed to fix the issues? Let us know in comments.