Hmmm, interresting concept - silent PCs.
It's when I can't hear my pc that I know there's a problem. If a fan stops spinning, I'll notice it if it's normally loud.
Being a smoker who smokes in front of his multple pc's, hearing the fans at work is a good thing. When I can't hear them, or when smoke dust has worked its way into the bearings and they begin to whine on startup, that's when I know it's time to bust out with a tooth brush and some oil to clean them up and "refurbish" them.
Granted, I do run motherboard monitoring software (Asus Probe, MotherBoard Monitor, etc) which helps me keep track of fan speeds and temps. However, I'm not in a nuclear sub, monitoring the former Soviet Republic's naval activities - silence doesn't matter much to me. Effective cooling does.
One of the nifty features of my Plextor DVD burner is the option to slow the drive's opening mechanism to one of a small number of preset speeds, to silence it even more for "when silence is golden" or something. I say, let that drive open at full speed, because it's not that loud to begin with. However, for those looking for silence in components like a DVD burner, I'd have to suggest looking at the Plextor line-up of drives. Mine's a bit old (it's a PX-716A), but it acts just like new.