Actually, I would advise that you have the pS3 situated in a well vented area. Do not have anything blocking any of the air intake places (the little squares that look like a radiator on the front of a car).
Make sure to regularly wipe away any dust that may be clogging these areas up as the PS3 attracts dust like nobodies business; like a picnic attracts ants.
I constantly have my PS3 running for days and days at a time. The only time it has been switched off over the last couple of months is when we have either had a power cut, or somebody has pulled the plug out by accident.
It does overheat, but by wiping away the dust blocking air getting into the system, the PS3 cools itself down and it becomes as quiet as a sleeping baby once again. When it's running quiet like it's supposed to, you may even find yourself checking that the thing is actually still on; that's how quiet it can get.
Hope that helped with problems with overheating.
Jay