Overheating Diamond on Idle

[marcusm2007]

I've done some searching around and see that overheating is really common when charging or talking, but.... my phone did it while it was in my pocket during class, not in a call and the screen was off. Also the battery had gone from 90%-20ish%

It was hot enough for me to get uncomfortably toasty before I noticed.

I already plan to buy a HTC Hero by Christmas, but do you think this is worth throwing a fit with Sprint to maybe get a discount or something?