Karl Schroeder points to a paper by Keith Wiley about the Fermi paradox and von Neumann probes. He converges on arguments made here, here and elsewhere:
1) that von Neumann probes are a better way to detect alien intelligences than electromagnetic radiation (either signals or detecting large structures)
2) that if self-replicating probes are possible and there are alien intelligences, there should be loads of them in each solar system already. The best feature of the linked paper is that Wiley takes a stab at estimating how many there should be in this solar system.
3) that the universe as we now see it could already have been profoundly influenced by the activity of intelligent entities, but we don't know because it's the only way we've ever seen it and we have dumb-matter (i.e. natural) explanations for everything. Apparently non-testable but still interesting to think about.