I bought a Linutop2 pc, specifically to test out the usefullness of a behind-the-tv computer, and I can confirm that it is a very comfortable and workeable concept. Linutops are Intel-compatible low-power devices that can be left on (a Linutop2 uses less than 10 watts of power running) all the time. I used to plug mine in the VGA outlet of a LCD tv, thread an ethernet cable to it and connect a wireless keyboard to it. A couple of USB sticks make for plenty local storage and presto! Your tv becomes a surfmachine.
However...
The Linutop is really underpowered for anything more than simple surfing, it runs unoptimized software (basically an unaltered Ubuntu edition) and it has no Bluetooth or WiFi on board. All this is already reason enough to dream about a PandaBox as a replacement. Add to that the emergence of a dedicated software platform and we're cooking with plasma.
Note about the keyboard: A stationary PandaBox should ideally come without built-in keyboard. I found a nice gadget from the good -- if often slightly deranged -- people at Brando. It's here:
Brando keyboard
I'm sure there are betters, but this one scores high on the cuteness scale. A BT version would even be better, but I can't seem to find a decent and affordable BT keyboard with built-in trackball or -pad. In any case, I can see a real possibility in getting this keyboard bundled with a PandaBox.
Of course, Craig was absolutely right when he said that a hypothetical PandaBox would require a hefty additional investment in testing and bug-hunting, which is why he proposed a simplified version: a "normal" Pandora, minus the top half (the part with the screen). Less innovative and more expensive to buy, but easy to produce quickly.
Personally, I believe this crowd is by now so used to waiting that they wouldn't mind sucking it up until a proper PandaBox could be offered. Still, I believe this might be a huge untapped market (behind-the-tv pcs that are always on and use insignificant amounts of electricity) with much pore potential than the "straight Pandora" market segment.