Even greater than the conspiracy to cover up evidence of UFO's by the US gov't is the conspiracy to cover up top secret research using a UFO coverup story.
I saw a show on the Discovery channel about the history of Dreamland (or Area 51 as it is popularly called). Area 51 was a box on a map of Nevada, right there between area 50 and area 52. It happened to have a large dry lakebed so when the top guys in the US air force needed a place where they wouldn't be bothered while they tested the U2 (spy plane, not rock band), they decided that Area 51 would be a good spot.
About the same time some air force guys in New Mexico were testing weather balloon designs outside of Roswell. The balloon crashed, a farmer found it, the air force reclaimed it, and the story of a weather balloon crash was reported in the Roswell newspaper the next week.
The guys at Dreamland (that's what others in the air force called it because they weren't allowed anywhere near it) decided that they needed a cover story to keep people away so they "accidentally leaked" that the weather balloon was actually a crashed UFO. The press caught on and faithfully reported the UFO story across the nation, and they made it sound like it was all a big secret, which is just what the air force wanted. People were too distracted by the UFO story to care about Area 51 and the U2 prototype. And over the years as the air force has tested the stealth fighter and stealth bomber (and other unknown airplanes) at Dreamland, people have mistaken those planes for UFO's and the air force lets them. If people believe that secret spy planes are actually alien spacecraft then national security remains intact.
Well that's what I think anyway.
But there are surely other intelligent species in the universe. And while VWguy is right in his calculations, it must also be understood that the distances between these civilizations must be immense. I think it's unlikely that any other intelligent aliens have visited Earth because they would have to be a lot older than us to possess the necessary technology to cross interstellar (and even intergalactic) space. The universe isn't as old as we think it is; other systems that can support intelligent lifeforms probably aren't much older than our own. And if you ask me, I say that we're probably the oldest civilization in this corner of the universe (well, this corner of the galaxy, anyway).
I learned of an interesting theory (in a scifi book, I think) that says that evolution on Earth has been occurring at an accelerated rate. Why? Because of Jupiter and the nearby asteroid belt. While Jupiter does have a tendency to eat up dangerous asteroids, it may be to blame for some of ancient history's worst asteroid collisions. Jupiter causes perturbations in the asteroid belt that sometimes results in asteroids being flung towards Earth. Two major collisions have been recorded that had serious effects on Earth's ecology: one 65 million years ago that resulted in the extinction of the dinosaurs, and one some time after that that may have caused the ice ages. Obviously there must have been asteroid collisions before those two that we don't know about.
But the point is that every time a global ecological disaster occurs, the less evolved species die out and the more advanced species adapt and multiply; hence the accelerated evolution. True, there may be innumerable planets elsewhere where life may have evolved, even intelligent life. But unless their planets have been subject to such disasters as our's has, it is unlikely that intelligent life could evolve as quickly as it has on Earth.
Or maybe not.
The questions of the existence of extraterrestrial intelligence are very intriguing, but what I'd really like to know is why human beings continue to insist that they must be inferior to other alien species. People might be egotistic sometimes as wench points out, but usually we agree when people tell us we are insignificant. That's really sad. Intelligence is a rare and unique thing (even among humans) and we should acknowledge that fact and be proud of our place in the universe. At least we're not as arrogant as the people of Krikkit from the book Life, the Universe and Everything (R.I.P. Douglas Adams) who were so sure that they were alone in the universe that when they discovered they were wrong they decided to destroy it.