Sometimes when I chat with people about UFO's and such (by the way I don't believe that the Earth is being visited by Aliens, I think something much more sinister is going on then that), one of the immediate fall back positions that people often use to discredit what I will call "the phenomena" goes along these lines...
"If there really where aliens visiting us, and the government knew about it, someone would leak."
My response is often that there are people who leak. Here is a recent example-
Again, I think UFO's and Alien Abduction type stuff fall into the spiritual realm. In my opinion if anyone takes a serious look at the phenomena, they will not be able to easily discredit the notion that something (or somethings) is going on, what that something is (or somethings are) is what the serious debate is all about.