IMHO it's again a problem of probability, if even one in a million planets with life develops intelligent life, and even if the average lifetime of intelligent life is only one million years, that's still MANY civilisations out there. Some of them (even with the hypothesis that you cannot exceed the speed of light) might have grown to become space empires.
And considering the timescales of the universe, if there's at least one space empire in our galaxy, there's a good chance that at least some of their space probes already know about us. Now, I really doubt that if this is the case, that they will bother with all the crap the "UFO-green men-conspiracy" people pretend they do - unless, as I have stated in the IRC, their favorite form of sport is "Confuse the Earthling".
