Okay, that's a bit alarmist. But Stephen Hawking has generated a bit of buzz by pointing out that contact with an advanced alien civilization might not turn out well for us backward humans. In fact, we should just try to keep quiet and avoid being noticed.
"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. Prof Hawking thinks that, rather than actively trying to communicate with extra-terrestrials, humans should do everything possible to avoid contact. He explained: "We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."
To which I can only say: yeah. Sounds about right. If aliens were sufficiently enlightened to be utterly peace-loving and generous, it would be great to have back-and-forth contact with them. But it's also possible that they would simply wipe us out -- not necessarily in a Mars Attacks! kind of invasion, but almost without noticing (as we have done to countless species here on Earth already). So how do you judge the risk? (Dan Drezner gives the interplanetary-security perspective.) It's like the LHC doomsday scenarios, but for real -- the sensible prior on "murderous aliens" is much higher than on "microscopic black hole eats the Earth." Happily, a face-to-face chat seems unlikely anyway. Nothing wrong with listening in, on the unlikely chance that the aliens are broadcasting their communications randomly throughout the galaxy. Besides, a little advance warning wouldn't hurt. Update: I had forgotten that we had already discussed this a couple of years ago. Old bloggers tend to repeat themselves.