Paul Davies speculates that a space-faring civilization could use nanotechnology to build miniature probes to explore the galaxy, perhaps no bigger than your palm. Davies says, The tiny probes I'm talking about will be so inconspicuous that it's no surprise that we haven't come across one. It's not the sort of thing that you're going to trip over in your back yard. So if that is the way technology develops, namely, smaller, faster, cheaper and if other civilizations have gone this route, then we could be surrounded by surveillance devices.
Furthermore, the development of biotechnology has opened entirely new possibilities. These probes may act as life-forms, reproducing their genetic information, mutating and evolving at each stage of reproduction to enhance their capabilities, and may have artificial intelligence to accelerate their search."
This argument, plus the arguments in Rare Earth, leave little doubt that we are alone at least in this galaxy and perhaps in the universe. I would put the upper limit of intelligent species in the galaxy at no more than 100 and possibly no more than ten.
Otherwise we would be having visitors (large size, the guys who built the nanorobot probes).
The Fermi Paradox.
--Boris
Definitely went against the grain when it was published. However, it makes a lot of sense. A lot more than most wishful thinking that is going on in this area.