I think mCubed is the only company that has understood and provides true silent computing today. And by silent I mean passive cooling. I have built a system using their old classic cabinet and has been very satisfied with the performance of this. The only detail I did not like was the dreaded plexiglass alignment of the optical drive as well as getting the eject button to work (mine would hold in the button on the optical drive all the time). Other than that the case performs as it should and I have a friend who is passively cooling a GeForce 7950GTX in this as well - he has an awesome gaming rig in his livingroom - dead silent! Its truly something you have to experience to appreciate. People are just too used to noisy computers that they dont see the problem anymore.
Well I am never going to build noisy systems anymore. My next office desktop system will be a 100% passively cooled one too. Chances are that I might even get another hFX Classic for that even though that is really made for a HTPC setting.
About the optical drive, there is some software on the market where you can set the drive to operate silently - I use AnyDVD which works brilliantly for this. Watching a DVD or listening to music, you dont have that terrible whooshing sound. Its truly dead silent.
Also, in 2 years time Samsungs Solid State Disks (SSD) will have come down in price and you can have the full operating system and necessary applications on a fast, power efficient and totally silent SSD drive. This removes the last noisy part of your HTPC. All media, music and movies you can store on a noisy harddrive in a server packed away in a storage room. I am really looking forward to these new SSD drives.