hmmm well I've finally bought that NoiseBlocker S3 fan I was told was the best deal about dBa/cfm ratio, but actually it's not THAT noiseless... my CPU fan makes less noise that that (it's an Arctic Cooling Super Silent 2).
The good thing is that the NoiseBlocker comes with a potentiometer that enables you to slow it down a bit. And around 2000 rpm the noise level drops down to something very acceptable. It's however much more silent than any normal case fan.
And I like its color too... blue fan made of translucent plastic.. l33t
*edit*
here's a pic on how it looks like
