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As far as I can tell, the most quiet mac is the mini. If the new Imacs are anything like my G5, you must be deaf not to hear the fans under any conditions, but especially with heavy tasks. When you're just typing text it's fairly quiet but not dead quiet by any means.

(you get used to that noise of jet engines gradually - I have, but it's not the most pleasant computing experience)
 
I was doing a study today with a previous-generation cinema display and a G4 cube behind it. I could hear the hard drive spinning in the cube, nothing else. I couldn't believe how quiet it was.
 
I have a Duo iMac 2.0 and its 15 months old now. When I first got it, it was whisper quiet. now its getting a little louder each day.
 
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