the cabinet argument is a balancing act, to me the only reason to hide an HTPC is either because its an ugly scrap build of random parts put together just to get access to media, or because you just want to get it out of the way in an effort to cut down on noise. A system like this you could put on display next to your receiver (if you are the type to have that on display) or next to your gaming consoles. I know very few people who hide their consoles for fear of them overheating or just because it is easier to keep them handy for swapping disks.
But personally I have had my HTPCs housed inside an isolated cabinet from IKEA for years, and a variety of builds ranging from my original e1200 celeron up to my current 2100T + HD 6400 (2gb/128-bit) and though I've added some ventilation for the sake of having weekend projects overall I've never had any real issues. Originally all I had was just a 120mm hole on the rear of the cabinet that I used for the wiring to the receiver and the only fan in the original build was on the e1200 (HD 5450 I had paired with that was passive).
So if you had sizable passive ventilation, I'd wager this would do just fine behind a glass cabinet door. Modern systems do quite well at throttling at idle and in sleep mode to cut down on heat compared to older systems. And use an SSD in here, and keep your media on a NAS somewhere to avoid mechanical storage dumping heat into there as well. The OS SSD in my server typically runs a solid 5-10c lower than the mechanical storage dumps, even the 1TB Raptor in my main rig runs between 33-38 vs 29-35 of my 240gb Kingston HyperX. So you can imagine the savings would be much greater in a case this size compared to a Corsair 600T that has a 240mm fan right in front of the hard drive rack.
But I really am anxious to get my hands on these new APUs for some HTPC builds, and see if I can shave some watts off my current system. 2100T + HD 6400 is pretty efficient, and I'm really disappointing AMD didn't release any 34/45w TDP parts, but I guess some later binning or undervolting/underclocking might get us there without any appreciable performance hit.
*edit*
Also, you'd wonder if they would release a heatsink adapter that would let you passively cool a GPU that you could hybrid crossfire with the IGP using the cooling fins on the other side of the case?
AMD recommends using an HD 6670 with an A10 series APU, perhaps that could be passively cooled?
Sapphire and
HIS both have dual slot versions that are passive, but I guess you would have to share the CPU-side heatsink to get that same kind of surface area...