There is no backplane for the CPUs no. But they support 10 and 25Gbps networking that can be used for high speed clustering.

Now that you mention that, it's quite an upgrade over my idea of putting 32 Octavo easy to interface packages (you need to connect like 5 balls for what I want) along with a ethernet switch and make a cluster of 32 puny nodes...

The number 32 is important because a stack of boards should look like a Thinking Machines CM-2a (http://www.corestore.org/cm2a.htm) and each CPU would control one LED (or three - Octavo now has a 3-core, two puny, one even punier, part, ending up with only 10 nodes per board).