I'd think that the phone has just been charging for to long. Or maybe the surface the phone is on while charging has something to do with it? I'm not positive, but thats what I'm thinking.

Please answer my question:
http://answers.yahoo.com/question/index;_ylt=Al7Q17LUATk7eqf3th7kEci3xQt.;_ylv=3?qid =20080716012132AA3TYRn