DARPA is building test bed for virtual satellite clusters. More here.

The 'building block' concept is certainly interesting, especially in the sense that they system is easily reconfigurable and there is little chance of single-point failure, but now there are complexity issues. Each building block of the constellation has to have its own power, propulsion, communication systems, etc., where a single satellite does not.

It's not clear to me that the advantages of clustering outweigh the disadvantages.