Why not white box PC it with some of the new low power low profile gear available? I can totally understand the esthetics of the whole thing, but some of those cases now are pretty nice.
Myself I have an old Dell desktop hiding behind an easy recliner with cables running along the floor up to the tv including a Microsoft MCE USB remote dongle. It is not pretty and will one day build something new to put in the rack. Ether that or figure out the back end stuff so that I only need to use a game console.
Isn't the main advantage of the Mac Mini its small size? You kind of lose that if you are hanging two HD-PVRs off of it since they are each substantially bigger than the Mac mini, not too mention the cable boxes and the bulky component cables.
The best way to do this is to have all of the boxes hidden away in the basement and to use an extender of some sort at your TV(s).
Here is my setup with two HD-PVRs, two HD STBs and one SD STB - it ain't pretty but it is in my "server room" so who cares. And this photo doesn't even include the PC.