Given the number of Xbox360 hardware failure attributed to overheating I wouldn't have thought your was stored in a great place anyhow! It appears to be pretty important to allow the air flow through them and this doesn't like it'll get any airflow.
I speak from experience ... mine died tonight
I stored it on top of the Sky+ box which I only got recently, and suddenly I start hitting problems. I strongly suspect I overheated it with it's placement.
Jed