And @Alway: I saw a program on Scifi about how a 'real' war with aliens would go, and the humans ended up using those devices as giant nuclear coin guns. It was kinda epic.
Uh, no... A real war with aliens would go something like this:
1. undetectable probe arrives in system
2. probe waits and collects data; perhaps for millennia; it could already be there now and could even have been there since before our ancestors walked upright.
3. aliens or automated probe protocols decide to wipe us out
4. undetectable nanobots (though in all likelihood, they could be on a pico or femto scale) are dispersed in the atmosphere
5. nanobots activate after dispersal, with effects ranging from instant death of all life to initiating planet-wide terraforming, depending on the goals of the probe.
Sheer statistics dictate that if we do encounter an alien race by them visiting us, they will be so technologically advanced relative to us as to make resistance not futile, but nonexistent. If they wanted us dead for whatever reason, every last human would be dead before we even had a second to realize something unusual was going on. Keep in mind, the universe is about 13.7 billion years old; we've been around as a species for only a few thousand, and only the last few hundred as a species capable of rapid technological progress. If an alien civilization found us, it may well be a billion years older than ours; it may have had millions of times the amount of time we have had to develop technology. We're talking top of the tech tree here (assuming it isn't turtles all the way down).