Wolfgang, an addendum to your (accurate) description of the problem. There's another dimension to the equation - time. How long can a species live for? How long does it take to develop a technological civilisation? How long does a planet even remain habitable? Or a solar system or galaxy, even? Given that, we can work out the probability that two technological civilisations are going to occur roughly at the same time, and close enough together that they can communicate - otherwise, all we'll ever find on other planets is fossils (or maybe all the aliens wil find will be our fossils).

The problem for these equations is that even taking the most wildly optimistic numbers anyone can suggest (anyone who's expert in the appropriate field, I should say, not a wide-eyed amateur like von Daeniken), the odds still come out pretty damn small that we're going to meet an alien.

Another aside - following "Close Encounters", alien sightings have apparently pretty much standardised on what they look like. Go figure.

Grab.