It's unusual that a system with a fan is so quiet. But i've heard that one can't even hear the fan when one's ear is against it.
My wife has the 2010 model (Core 2 Duo) and I recently bought the 2012 model (Ivy Bridge). Both 11" models.
The 2010 model is very quiet. Idling, surfing the web, watching videos is all done with fan on minimum speed. It is barely audible in a very quiet room. But just faintly. Even my SPCR-spoiled ears are perfectly happy with this sound profile. The fan tends to ramp up if the battery is charged while doing work, usually to audible but fine levels.
If you really stress the CPU (for my wife this is typically a sign of an app going berzerk) and/or charge, the fan becomes obnoxious. Not the worst I've heard, surely, but really bad.
But 95% of the time, the MBA 2010 was the quietest laptop I had ever heard.
Until I got the 2012 model. This one is just insane. Here, 95% of the time I cannot hear the fan AT ALL. It is running, you can clearly hear it when you hold your ear against the case, but in a normal 1'/30cm distance to the laptop it's inaudible. And it doesn't. ramp. up. Multitasking while charging the battery does not make it ramp up or.
Only prolongued CPU load, such as unpacking multi-gig archives, will accomplish that. I've only had it happen 3 or 4 times in 2 months. Even then, the fan ramps up to "just noticable". It's about 1/10 the loudness of the 2010 (pure guesstimate of course).
The 2012 MBA is almost a perfect laptop for me. The keyboard is gorgeous. I've been keeping an IBM Model M around on my workstation PC. The feeling is obviously totally different, but I find the Mac keyboard to be better.
The touchpad is glorious, battery life for the 11" model is about 5 hours of light use, which is fantastic. Only the screen is mediocre. Here I wished they would have gone for an IPS or PVA, like Asus and Samsung did with their recent Ultrabook greats. Viewing angles are pretty bad with these TNs. It's bright though and there's almost no glare - the exact opposite of Apple's glass front Macbook Pros.
I was tempted with the Zenbook Prime (the Samsung Series 9 is unreasonably expensive and only available in 13") for its IPS glory. I'm curious if it has similar sound characteristics. It uses the same hardware and a copied chassis design.