This topic is on my mind because I've just recently read two good SF books that touch upon the idea: Alastair Reynolds's Pushing Ice and Jack McDevitt's Odyssey. (Both recommended, incidentally.) On the surface their plots center about different issues, but lurking in the background of both is that same old question: What about the aliens already? Where the heck are they? Do they even exist, or are we alone?
John von Neumann, a pioneer of computing, put forth the idea of the self-replicating machine in 1966, and this is directly relevant to the question of whether alien life exists. Nowadays often referred to as von Neumann probes, such machines (von Neumann suggested) could be used to explore or even colonize the galaxy. A single probe equipped with a factory capable of building an exact duplicate of itself could fly to a target system, explore it and/or exploit it in any way it chose, then build copies of itself out of materials found in the local asteroid belt and send them out to neighboring star systems where they, in turn, would explore/exploit and then build more probes, and so on.
Frank Tipler suggested that this is logical proof of the non-existence of alien life, because -- even if you started with just a single von Neumann probe and take it as read that faster-than-light travel is impossible -- you could establish a presence in every star system in the galaxy in a few short millions of years. The Solar System is not under alien occupation and there is no evidence that it ever has been, ergo this has never happened in the billions of years the Solar System has existed, ergo it is very unlikely that aliens are out there.
(Those out there posessed of exquisite taste may recall that just such a recklessly self-replicating robot probe is one of the first hazards you encounter in the classic game Star Control II.)
Realistically, the details of whether one uses a robot probe or not aren't important -- what's important about Tipler's argument is that, given that it is possible to colonize the Milky Way Galaxy, and that said Galaxy has been around for around twelve thousand times longer than humanity has been walking upright, and that yet we see no colonies, then the lack of alien colonies in our Solar System is a powerful argument that nobody's out there.
There are, however, a few cracks in the argument.
The smaller, more depressing crack is the argument assumes it is actually possible to colonize the Galaxy. Suppose the technology is simply too difficult? Or every civilization, once it advances far enough, will inevitably collapse, destroy itself, or transubstantiate somehow? Or generate its own version of Senator William Proxmire to shut down the space program because we need to solve our problems down here on Alpha Cephei IV-C first? The result could be any arbitrary number of alien species, effectively imprisoned in their own star systems. (Though this is a weak argument, ultimately, as it assumes an absolute ironclad rule applying to a wide variety of alien psychologies and cultures. Nobody in twelve billion years gets it together for long enough to climb off their homeworld? Really?)
The large crack is that "we see no colonies." If there was an alien ski resort on the back side of Sedna, we'd have no idea it was there. More generally, we have never detected a radio transmission from another planet, but then we haven't exactly looked all that hard. We don't have the ability yet to examine other star systems closely enough to detect signs of technological civilization (though within the next twenty years that may change.) All we can say for sure is that there's nobody here, in our face, right this second, and if there ever was anyone here in the past they picked up after themselves carefully.
Putting it all together, here's the plausible alternatives I see, going from most calm to most crazy:
1. Tipler's conclusion. There really is nobody out there and the Galaxy's ours if we want it. Neat, clean, and simple, and whatever meaning the Universe has is what we will bring to it if we can pull ourselves together and make it happen.
2. A position very similar to Tipler's in some ways: consider that if intelligent life is very rare, a culture that colonizes hundreds of thousands of worlds and lasts for ten million years might still not encounter any other living cultures in what is really still a brief span of time compared to the past and future age of the Galaxy. Our ships would go out to the stars and find nothing but unimaginably ancient ruins, and presumably the same would go for the next bunch of guys two hundred million years from now except our ruins would be added to the pile. If we want to talk to someone else we'll have to find some way of waiting around until they evolve. Alastair Reynolds has played with this one, and McDevitt uses it on a much smaller scale as none of the aliens seen in the "Academy" series manage to really spread very far before succumbing to one sad fate or another.
3. Intelligent life is common but some malevolent force, natural or unnatural, prevents it from colonizing the Galaxy. One might imagine robot battle fleets or Omega Clouds, but such a force could be quite subtle. Picture a Von Neumann colony hanging out in the Oort Cloud around every star, listening for radio signals indicating that some poor luckless species has discovered E/M communications. That's their cue to nudge a few comets out of their orbit towards the source of the problem. Given how long we've been happily broadcasting radio in all directions, and how long it takes for comets to get places... um... better not buy any green bananas!
4. Intelligent life is common and has colonized the Galaxy, but for some reason deliberately leaves our star system alone. One could imagine any number of reasons, from an elaborate non-intervention policy to a society like in The Hitchhiker's Guide to the Galaxy that's just too cool to hang out with the monkeys. No matter what, though, it implies a sophisticated civilization that has existed for a very long time and is capable of enforcing its point of view but which at least doesn't seem to want to destroy us outright, so that's good? Maybe?
This is truly one of the most fascinating questions that exists, because every possible answer staggers the mind.