We're on the edge of a bold new era in space exploration. In the next few years, Kepler and other probes like it will add more and more exoplanets to our maps of the galaxy, potentially even exoplanets the size and general nature of Earth. If there is a life-bearing world in a nearby system, it will not be much longer until it's found - there are no shadows or horizons for it to hide behind.
For what it's worth, I hope that we do not find any evidence of active technological civilizations, and for one simple reason - because there are no shadows or horizons in space for anyone to hide behind. The belief that an advanced extraterrestrial civilization would have to be friendly by dint of its survival is not necessarily true. If nice guys finish last, it's because bastards have a tendency to come in first - because bastards are ruthless enough to do what they need to stay in first. Even here, we have a reflection of Earthly politics.
Relativistic weapons are something that aren't commonly seen in science fiction - I'm not sure if that's because the concept is a fairly recent one, or if most authors just don't want to think through the implications of what Einstein figured out - but the more that I think about them, the more I believe glossing over them is a mistake. To put it simply, a relativistic weapon is the ultimate bullet. Take a probe, an asteroid, a small rock, anything, and accelerate it to a high fraction of the speed of light - say, 0.99 c or somewhere in that area. At that speed, relativistic effects would result in the mass of the projectile being far higher than at relative rest. It would also have a truly ferocious reserve of kinetic energy behind it.
What's more, it would be as much of a "bolt from the blue" as is possible in space. At that speed, the projectile would be nipping the heels of its own light, as well as any radio reports from potential forward observers. The first evidence of its approach would be when it was practically on top of its target - little to no warning time. A relativistic weapon would, in my estimation, be the weapon of choice to destroy a planet, if necessary.
There are some who think that extraterrestrials would consider it necessary. Tom Ligon's "El Dorado," which appeared in the October 2007 Analog, is based around this concept. In that case, the weapon is only deflected from its target through extreme good fortune. Ligon's extraterrestrials attacked out of a religious motive, but one could just as easily imagine a species who sets aside a stock of relativistic weapons as insurance - to destroy anyone who could knock them off their pedestal, just as soon as they realize there's someone around who might, conceivably, at some distant future time do the knocking.
Yet, that also struck me as insane. Destroying alien worlds because they might, conceivably, at some point become a potential threat? Beyond the sheer moral concerns, there's also the issue of incomplete intelligence - a species that launched a relativistic missile could well be sealing its own fate. A relativistic missile could be deflected - most likely by chance, but there's always that chance. Even if it succeeds, there's no guaranteeing the target doesn't have its own stock of relativistic missiles, and that a dying species' last act would be to press the button that would annihilate their murderers a few years or decades down the line. There could even be a third power, unknown to the attackers, who would destroy the attackers' own worlds the way we would put down a rabid dog.
There's nothing for planets to hide behind in space. That's one of the reasons why I hope that we're alone in this particular area of spacetime. Of course, relativistic missiles lose most of their teeth if faster-than-light travel is brought into the equation - sky surveys from multiple vantage points, separated by multiple light-years, should be able to detect an accelerating relativistic weapon well before it reaches the target.
On the other hand, the way our current understanding of the universe goes, we can either have FTL or causality, and our scientists seem to prefer the latter. Personally, however, I'd rather have a universe where effects can precede their causes under the proper circumstances than a universe where aliens can exterminate entire worlds because they don't like the crackling of their radios.