It's very important to keep it out of an enclosed case of anykind where hot air can move and escape freely from the unit, I have mine on a A/V shelve that isn't enclosed and the fan has never come on despite running it for up to 5hr with movies or games.
I think mine must be unusually loud. It's in an entertainment unit, but has plenty of headspace and space around (about 1" all around, and open back). The fan comes on pretty much right away, and is noticeable on anything than a loud action movie. Maybe I should call Sony.
My PS3 is also in a cabinet with about 5" all around and vent cutouts in the back. With the front doors slightly ajar for air circulation the PS3 fans come on full blast at around an hour into a movie. At 8 feet away inside my cabinet It was loud enough to distract me at quieter scenes.
My solution, added a 4" computer fan into the back of the cabinet to help increase air flow. The PS3 is now always quiet and the added noise from the computer fan is minimal.
edit: That's weird, my post went into the wrong thread. Admins please move to "Noisy PS3". Thanks.
My 80 gig has fan noise from the moment you turn it on,this is my second unit as the first the fan noise was audible from 10 feet away.I have it on top of my center channel speaker with plenty of ventilation.The second 1 is a lttle bit quieter but not by much,overall though I like the unit.