This hands on from a month ago claims it's insanely quiet, but that's hyperbole as there's really no way for them to reliably tell in that environment. Interestingly they note that the outside case of the nMP gets quite warm.
However I think we can be assured that it will be extremely quiet. Apple says it has the same idle noise performance of the Mini - about 15 dB if I recall. This means that if you have it behind a monitor you won't hear it in a very quiet environment. I have a mini in that configuration that still has a spinning hard drive and that's the only component I can hear.
Given the design - the only moving part is an extremely large 6" fan pulling hot air upwards - I think it will actually be astonishingly quiet. I'm a computer noise freak, having worked with them so long I'm driven crazy by
any noise from these components. My old MP - a 2009 Quad core, is pretty quiet for a workstation but right now I can still hear a low hum from it, even though it is on the floor. And occasionally I still hear it access the CD rom for some reason (a small click sound). Luckily games have a sound so when the GPU fan spins I don't notice it above the game audio.
The other aspect is the nature of the noise. When you do hear this it will probably sound like a dull "woosh", so may actually be somewhat pleasing. My old Mac Pro doesn't have a lot of high frequency content in it's sound thankfully, but those five odd fans and the one hard drive do make a muddy sound that gets on my nerves eventually.
This is probably the main reason I'm looking to get a nMP - for software development I want a completely quiet computer and I expect this to be it.
