Okay, I read Wendy Davis' recent post, and I had a diary-length extended comment.
It's amusing to see the US government take UFOs seriously, that is, if Jeremy Corbell is to be taken seriously. Since I cannot say for sure if any UFO speculations are true or false, let me suggest some preliminary background through which the idea of an "extraterrestrial civilization" visiting us need to be digested.
1) We can guess that there are a certain number of extraterrestrial civilizations in the universe, and we can guess this from the extremely large number of stars in the universe. Our galaxy is estimated to have 100 billion stars, and there are about 10 billion galaxies in the observable universe, so here's the math on that one. Now, of course, to be like our Sun, a star with an Earth-like planet would have to be a certain distance from the center of its galaxy, so that limits the matter a bit. It would also have to be a certain size and lifetime as a star. And to harbor intelligent life like, well, us, such an Earth-like planet would have to have the right conditions -- gravity, length of day, appropriate axial tilt, appropriate atmospheric composition, oceans with water in them, appropriate strengh of tides, and so on. Much of that discussion is covered in an old book by Stephen Dole: "Habitable Planets for Man." I read it when I was a kid.
2) The stars are too far away. It takes light 4.3 years to travel from Alpha Centauri to those who can see it in the Southern Hemisphere; for the next pleasantly Sun-like star, Epsilon Eridani, we see it as it was 11.1 years ago. Neither of those stars is going to have planets with extraterrestrial civilizations. So we must look further out, and imagine that any such civilization wishing to travel here must find some way of getting here that accounts for a journey time of decades (even given Einsteinian time dilation) and the significant probability that something could go wrong in between their planet and ours. The likelihood, then, is that the extraterrestrial aliens are just too far away to visit us.
3) Even given the likelihood of life elsewhere in the Milky Way, it's highly unlikely that such life would evolve anywhere into beings like us. Let's take a look at some of the prerequisites for our existence:
a) Earth -- we've been over much of the problem of the existence of Earths in the universe
b) life in bacterial form
c) cyanobacteria, reshaping Earth's atmosphere so that it was defined by the presence of oxygen and carbon dioxide
d) the aforementioned forms of life surviving extinction events, both of "icecube Earth" and of the possibility of a runaway greenhouse effect.
e) the proliferation of multicellular life in the Cambrian era, 541 million years ago
f) the extinction events after the Cambrian era themselves, which indirectly created the conditions wherein human life could have a niche
g) the various evolutionary events which produced people -- animals with opposable thumbs, bipedal gait, nutritional needs, sophisticated linguistic abilities, and brains capable of lifelong epigenesis, neural plasticity, and behavioral versatility
Take away any of those phenomena, and the Solar System wouldn't have people. The fact that we exist at all is due to the likelihood that very, very, very improbable things will happen in a really vast universe.
(We haven't even gotten around to discussing, here, the rather improbable events -- the inventions of philosophy, science, and technology -- that have allowed us to dream of extraterrestrial intelligence in the first instance. We might also discuss the rather low probability that advanced civilizations will survive over geological time -- the speculation that they haven't survived over geological time being part of the speculation for why we haven't found them.)
Of course, the vastness also serves as a drawback -- it might mean that others like us exist out there and that "we are not alone," given that there are so many billions of stars and planets, but it also means that those others are prohibited by virtue of immense distance from ever visiting us. No wonder SETI hasn't discovered anything yet.
So I really doubt it. Here's a novel thought: let's try to figure out a way of cutting out the distractions, and focus upon what to do about climate change. The password to open the PDF is: AddletonAP2009 . After all, life is not Arthur C. Clarke's Childhood's End, and the aliens are not going to save us.