I was pondering this a few weeks back, and after some thought I realized it's even later than I thought. The logic that an alien civilization should smash its neighbors before it can be smashed doesn't just hold for intelligence, it holds for life; if we can go from metaphorically crawling in the mud in the early 20th century (before radio) to potentially firing our own relativistic projectiles by, say, 2200, in 300 years, who's to say there isn't a species that could pull that trick in 30 years instead of 300? (Maybe somebody who can work together even better than we can, living with a faster biochemistry on a hotter planet? Something like Eliezer's "Maximum Fun-Fun Ultra Super Happy People"? Who knows? It's hard to imagine the human experience has been truly optimal on the technological development speed front.) In which case you may literally have no time at all to try to pick up the "intelligence". You need to smash all life, before you think it might have "intelligence".

And Earth has been broadcasting "I'm alive!" for at least 2.4 billion years, since the Great Oxygenation Event: http://en.wikipedia.org/wiki/Great_Oxygenation_Event Ever since then, any intelligence within the area of probably several hundred light years or more has had the ability to take a simple spectrograph of our planet, observe free O2 in the atmosphere, decide that's too likely to be the result of life for comfort, and obliterate the planet.

Nobody has. (Unless you believe the official Current Earth Destruction Status: http://qntm.org/board However, it appears to simply be instrument failure. Somebody really ought to fix that.)

Even on the galactic timescale, 2.4 billion years is not chump change.

Either the game theoretic optimum is not to smash your neighbors immediately (for instance, widespread orbital colonization is indeed practical and is what happens, which as people have observed changes the balance away from "smash"), or we are effectively alone. Either way I think we can discard the relatavistic-projectile-death scenario, except as a major, unplannable fluke.