You've all heard of the Drake Equation, a little exercise in rough estimation which attempts to approximate the number of intelligent, technological species in our galaxy. Here it is, if you haven't:
N=R* x fp x ne x fl x fi x fc x L
R* is the average rate of star formation per year in our galaxy
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets
fl is the fraction of the above that actually go on to develop life at some point
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.
I think it's an interesting exercise; even though most of the terms are rough guesses, nothing more, when you stack them all together you soon discover that even the most charitable estimates suggest that we could at best be dealing with only a handful of potential equals in the whole galaxy, and the galaxy is a very, very large place. (I'm not one of the charitable estimators: I'd guess very low on those last 3 parameters, and get an N of less than one.)
Drake is interviewed in Spiegel, and he leans toward the optimistic side, in more ways than one. He seems to think that any aliens out there would have matured out of primitive hostility (or survived selection for cooperation), which I don't buy. I suspect that if the human race could leave our solar system, we'd be a rather rapacious horde in our little corner of the galaxy, so I think that human temperament and history argues against that hopeful wish.
Anyway, another tidbit that I think argues against benign aliens: intelligent species would also carry a load of delusions, just like us.
Drake: Actually, one of my worst nightmares is that we find a signal and it will be an advertisement for a religious cult.
SPIEGEL ONLINE: Why would that be a nightmare?
Drake: I want to learn more about a civilization than just its belief in the supernatural. Religion is an important part of the culture but may not help to improve the quality of life in a civilization. Maybe their religion is a really good one, but I doubt it.
SPIEGEL ONLINE: That makes it sound like you're not religious.
Drake: I am not a religious person.
Everyone chant: One of us!
Now that that's out of the way, the possibility he advances is a horrifying prospect. If religion is a natural and intrinsic part of a sapient's way of thinking (which I don't believe, by the way, but I'm often told by the pro-religion camp that it is), it's another reason to hope we never meet an extraterrestrial, space-faring species. I'm pretty sure they wouldn't believe in Jesus, Mohammed, Vishnu, or Xenu. And all I need is space missionaries knocking on my door on Saturday mornings.