How serious does an asteroid threat have to be before we take action?

NASA's Near-Earth Object Program has revised the odds of asteroid 2011 AG5 hitting the Earth in 2040 to 1 in 500. While they describe this threat as "slight", it got us thinking: Just how serious does the risk have to be, before we should do something about it? » 6/26/12 12:00pm 6/26/12 12:00pm