When I was getting my 2 year-old son up from a nap, he opened his eyes, sat up, looked at me and said "Little green men" - as far as I know, a phrase he had not heard before. I assumed that this is what he had been dreaming about, and I decided to paint a picture of it.
Although in modern times the idea that Earth has been visited by alien life has taken on less of a divine connotation and has become interwoven with the idea of government or military deception (born of the Cold-War) the idea has been present throughout human history, and one of the almost universal features of mythology is that of celestial visitors bringing civilisation to a primitive human race, or indeed the human race itself.
This gives us little evidence for the existence of alien life or its involvement in human history, yet it remains an interesting and powerful theme in our mental makeup. It stems, I believe, from two features of the human psyche. Firstly, our desire not to be alone and to be part of something larger than ourselves. This itself a combination of man's essentially social nature and the fact that much of a human's life is internal, more than any other creature, and this leaves us with an underlying solitude and loneliness to our outlook. Secondly, a veneration of (and eventually deification of) our ancient ancestors, seen especially in ancient times as the source of all the 'higher concepts' of man that amount to civilisation and our elevation above the animals.
Bearing this in mind, I do not think that the idea of alien visitation should be instinctively ridiculed as the adolescent fantasy of bumpkins, but rather understood for what it is - an indication of a common human desire and psychological trend.
My own personal inclination about the question of alien life is that it is likely that life exists on other planets, since it would be a fantastic coincidence if the probability of life occurring within the universe led to exactly one planet with life but no more. I am unsure about whether intelligent life exists on other planets, as I do not know if intelligence is such an efficient feature of life that evolution would produce it frequently. The idea that alien life has visited Earth within the span of human history seems unlikely. Given the size of the universe and the time required for intelligent life to evolve, intelligent life would have to be extremely frequently occurring for it to coincide spatially (close enough to visit) and temporally (evolving at the same time) with that on Earth. The idea that alien visitations have been hidden from the general populace by a misguided government seems even less likely. If an alien civilisation were sufficiently advanced to travel to Earth, it is unlikely that they would need 'local help' to cover up their existence if that was their desire. Furthermore, given our frantic desire to meet another race, I think that if an alien civilisation were to find us, they would be equally keen to meet us, and keeping themselves hidden would not be in their interests.