Its been generally reported, outside some reports that updates have bricked certain systems, that the PS3 has been a reliable piece of gaming hardware, so a fan might not be a necessity.
overall those 'cooling fans' generally are a no-no cause the reason poster above noted and I believe if your system screws up cause of one of these the warranty doesn't cover(not sayin you can't lie but)
you'll never see a first-party cooling fan as the systems are designed to work the way they are. Third-parties trying to get some quick cash off scared consumers.