It was amusing to see discussion in the media based on some comments by Stephen Hawking (all good promotional material for his most recent TV offering). He raises the notion that it may be very, very bad for us if we ever come into contact with intelligent aliens. This is of course not exactly a new idea, but it can take on a number of forms - some more interesting than others. The Hollywood version is that advanced aliens might just be mean, hungry, and well armed. The version that I think is worth a bit more thought is to do with the slightly (only slightly) more realistic case where, rather than descending on us with hefty weapon-bristling spaceships, the aliens are simply communicating with us from afar.
Richard Dawkins coined the term 'meme' for a unit of cultural ideas or practices - and it's appropriate here - memes can propagate, much like a virus. An alien meme ('the universe is going to end in one year, and we have proof') could tap into our most lemming-like instincts and make it very hard for us to function as a species.
Presumably a sufficiently advanced alien species would understand the potential consequences of their communications, so would they still do it, or would they clam up and fall silent - neither wanting to damage other races, or risk being damaged themselves? For the sake of argument I'd propose that their best strategy would be to build an automated communications system, designed to engage any species unlucky enough to catch its whispers - but to never report back to the builders. This machine would be their meme-armor. It would effectively dispose of other civilizations in cases where the exchanged memes were bad, and keep the others guessing - perhaps it would even feign cultural collapse, to divert attention elsewhere. The clever aliens could go about their happy lives, having either wiped out annoyingly talkative neighbors, or seemingly dropped off the map.