Terrestrial Planet Finder is scheduled for launch in 2014, I believe Darwin is on a similar track. Budget deficits in the US are going to hit NASA space science pretty hard though, so maybe Kepler's success will be important to keeping TPF on track. Converse is also true if Kepler finds nothing.

I agree with Tom that discovering atmospheric signatures of ETL would be one of the most important discoveries in history. It would also hasten the development of even more powerful post-TPF scopes (as an answer to the "so what?" question).

Detecting and comprehending an ETI signal would be THE most important discovery in history.