If it is in a closed area it isn't well ventilated. The PS3 moves a lot of air, and will eventually heat up any enclosed space (my launch 60 gig plus a plasma TV heats my living room).

But even that hottest of the PS3 doesn't run the fan on high when watching BDs (grainy DVDs, and games will kick it up a bit). Mine sits on an open shelf.

Get some more air flow to the system.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris