I think that the idea of intercepting and decoding communications between ETCs is fundamentally misguided. The Powers are using data-compression techniques so powerful as to be indistinguishable from line noise. The human-level civilizations are using encryption techniques so powerful as to be indistinguishable from line noise - as John McCluskey points out, galactic economies are likely to be based on data exchange, so looking for unguarded information is like searching for 20 dollar bills on the streets of Manhatten.
What we should be looking for is byproducts - for example, a sphere-shaped region inside a galaxy that contains no stars of a certain type. Our telescopes aren't up to that, I think, but you get the idea. Does anyone have any ideas about what kind of involuntary signatures might be left by alien civilizations?
I still think the idea is fundamentally misguided, but it's less misguided than looking for deliberate signals.
-- email@example.com Eliezer S. Yudkowsky http://pobox.com/~sentience/tmol-faq/meaningoflife.html Running on BeOS Typing in Dvorak Programming with Patterns Voting for Libertarians Heading for Singularity There Is A Better Way