As a reply to some of those who quoted me, regarding the discovery and technological or differences:
Well, I don't believe in classical conspiracies, so I think ETs visitung us is like time travelling: if it were possible, it would've already happened, AND would would be a commonly known fact.
But even if alien races were capable of travelling interstellar or even intergalactic in reasonable time (so no "generations-ships" that travel for several hundreds or thousands of years), they'd first have to locate us. Which would be quite a feat, since every signal we've sent out there knowingly and unknowingly, only travels at the speed of light, and loses quality and discernability from the background noise in the universe with every lightyear it travels...
So even if our universe is sprawling with live, in most cases, those signals, which started to get sent out of our world about a century ago, would take millenia to reach planets with intelligent life, would have to be recognized as signals from intellligent life, and then those life form woul've to pinpoint our locations (try pinpointing a sepcific grain of sand on a beach for a similar difficulty), and then would have to mount an expedition, which again, would take, at least by our standard of knowledge, at best the exact amount of time to reach us the sign als took to reach them... highly unlikely.
Additionally, simple differences in communicational media would make detecting another life form quite hard:
XKCD - The Search [http://xkcd.com/638/]