I was reminded yesterday when speaking with one of the SETI board members of the very interesting conundrum we might find ourselves in if we in fact did receive a message from space. Above is some imagery from the first seriously high powered transmission from earth, dubbed the Arecibo message:
The  transmission consisted of a simple, pictorial message, aimed at our putative cosmic companions in the globular star cluster M13. This cluster is roughly 21,000 light-years from us, near the edge of the Milky Way galaxy, and contains approximately a third of a million stars.
So the best case scenario is that this data reaches a civilization in 21,000 years. Which means they think up some brilliant response, and then beam it back. Even if we found life much closer, it is most likely that the conversation delay would be at least a thousand years or more. So what does one say in a multi-millennial conversation? At least on the terrestrial end, each response would be made by wildly different civilizations.