This is a hardware issue, and google bring up some results from over a year ago, but would like to ask other 5230 users here.
My Nokia 5230, less than a month old, will occasionally get very hot while on standby in my jacket pocket. It seems random (had not used it for a couple hours, no apps are open), it just quickly gets hot for a few minutes. I've noticed this time that battery power has dropped by 40% after it cooled. (I don't attribute it to heat getting trapped while in my pocket, as I was standing outside in the cold when I noticed it.)
It warms after usage (camera, calls, gps, etc) but concerns me that it would also happen when idle.
Any other users experiencing this? I know a previous generation of battery was recalled in 2007, but this one (BL-5J) hasn't been, yet. Should be asking for a replacement handset? Or just a new battery? Or ignore it?