It's definitely a battery fault. I mean the only time heat is common is when charging, but if it's not charging then you SHOULD not have any problem with heat. Your phone is still under warranty, no?
I find that my BB gets hot when I run the GPS for a long time. If any of the radios are active for long periods of time, it might cause the BB (or battery) to get hot. So if you are running any type of IM or GPS application in the background, it may be causing the heat.