Got a dirt cheap 60 gig ps3 from Ebay, it was listed as turning off after 10 minutes. Had some demos on it so tested it and it did turn off after 10 minutes(also sounded almost silent) so I used some canned air to blow out of vents.
This time played Sonic Generations demo, both acts and it didn't turn off, then tried Rayman demo for about 15 minutes and didnt turn off.
And as a matter of interest turned the front of ps3 to face the ground(I assume thats the way to do it) and turned on, noticed a tiny bit of dust blow out back but no fan sound(I turned on from back then held eject)
Then took console apart and cleaned all the insides and replaced thermal paste with artic cooling MX-4
Put it back together but left top off case and then it worked but when I tried a 720p trailer it turned off after 2 minutes and had to leave it 10 minutes before turning back on
Didn't feel burning hot though, just warm.