The Bad Astronomy blog has had two good posts recently mocking people who think aliens have visited the Earth, here and here.
I've had posts on aliens before, so perhaps this is a good opportunity to clarify my position.
1. I think it's unreasonable to work off the assumption that humans are the only intelligent life in the universe. That strikes me as absurd. I quite firmly believe that alien civilizations exist in the universe, and I definitely enjoy speculating on their extent and nature.
2. I do not think it is likely at all that aliens have visited us in the last fifty years (when UFO sightings really picked up). I would wager that all the famous incidents - not to mention the obviously transparent ones - are naturally explainable phenomena, deliberate fabrications, or psychotic episodes.
3. However, unlike a lot of people that think the existence of aliens is reasonable, I don't think it's implausible that aliens have visited us in the last fifty years. The video in the second link, for example, makes fun of the idea that governments would keep aliens secret. I don't see that as entirely unreasonable, actually (perhaps if the blogger worked on any social science he wouldn't see that as entirely unreasonable either). Governments keep threats, sources of information, and new technologies secret from citizens all the time. There's very good reason for this (and sometimes not so good reasons as well, obviously). If there was an instance of alien contact I don't see why it's implausible that the government would keep it secret until they (1.) had a better understanding of the nature or the threat of the incident, (2.) were concerned about other governments making contact and reaping benefits, or (3.) were concerned that other governments would make peremptory strikes on R&D facilities if they knew we were developing advanced alien technology. None of this increases the likelihood that we were visited recently. I still think that almost certainly hasn't happened. It's simply to say that some of the seemingly (and suspiciously) convenient reasons why UFO enthusiasts can never confirm alien contact make sense to me.
4. If you think of the expanse of the Earth's history, a lot of Bad Astronomer's reasons for skepticism of visitation don't make sense anymore. That is to say, while I think actual visitation of Earth is highly implausible, I do consider various "ancient astronaut" theories more plausible than Roswell-type theories. That doesn't mean I'm one of those people that thinks the pyramids could never be built by humans. As a social scientist, I am constantly in awe of the innovation and abilities of the human species. I don't need aliens to explain anything I see. And I've never read a single page of Erich von Däniken. Nevertheless, when the ancients say "so and so came down from the sky, walked among us, and left" - I have no good reason not to entertain the possibility that "so and so" actually came down from the sky, walked among us, and then left. We really don't know either way. We have no way of confirming or refuting the account, and if we think it's reasonable to assume intelligent life exists in the universe we can't laugh off the idea.
5. I don't think it's unreasonable to think we may make contact with aliens over the next century (I do think it's unreasonable to expect that that is probable). We are obviously not going to get to them in the next century, but depending on how advanced they are and whether they're keeping tabs on us, they might come to us. Even more likely than contacting intelligent life, of course, is the chance that we'll find some tenacious fungus on Mars - which will settle the silliness about the fictitiousness of alien life once and for all.
Here, Michio Kaku provides very good reasons to (1.) think twice about many of Bad Astronomer's attempts at explaining why we should make a big deal of the fact that SETI hasn't found anything, while (2.) still being skeptical that they've actually visited us.
Having said all that, my mother-in-law's boyfriend moved out to New Mexico recently and my mother-in-law is moving out there this fall - we're going to visit soon afterwards and you better believe I'm taking a trip to Roswell while we're there. I don't care if everything suggests that nothing happened there. Roswell represents mankind starting to get serious about its cosmic significance (albeit, in a sometimes superficially unserious way - which is an interesting and ironic juxtaposition in and of itself... perhaps we need to be a little unserious to process such a serious underlying realization).
UPDATE: On the Colbert Report rerun last night, he interviewed Leslie Kean on her new book on UFOs. It was very interesting and one of the nice things is she seems to come from a skeptical position, and she's looked at more formal, official documentation of the events. Her point is (1.) most UFOs are easily explained, (2.) we can't say confidently what they are, and (3.) we also can't rule out that even recent events aren't extraterrestrials.