But I'm still curious exactly what the power is, what the resistance of the power connections are, etc.
As you said, it gets more interesting when they're not in a backplane, so I wanted to figure out what the constraints are.
Someone on one of their blogs linked to http://www.xmos.com/ which is also pretty interesting. It's a single chip with 4 cores, each of which can run 8 threads. And it takes care of all the switching between cores or even between chips.
It doesn't have all the same features as this board does, such as the dynamically switched power/signal pins, but same sort of idea.
Edit: In fact, the modularity of this is one of the more interesting aspects of it to me. I'd like to see how they're switching pins between signal and power dynamically. But, there's no schematics that I can find.
Basically, as the size of the grid increases so do the number of paths to an edge element. The practical limit relates to how much current you can drive through a single, element, but you can always use more than one power connector.
Edit: USB connection it can support a 3 x 3 grid (500 milliamps * 5 volts / 9) ~= 0.3 watts. So 130 of them should equal a low power CPU.
These are not going to suck down much power. They have around 1/1000th the processing power of a 3Ghz quad core chip and 1/250th the RAM. If you want to build a useful mesh like this then cooling is going to be the first concern, followed by latency. The best design would be a back plane with these cards inserted into the side. Add power at the top and cool down the sides with the back plane for communication. But that's what a standard motherboard with PCI-X slots look like so it's less interesting.