@jake-the-peg
The Drake equation contains an explicit probability of 'intelligent life' evolving. The problem however is that we only have one data point, the earth, which makes an estimate of the probability of life elsewhere entirely speculative.
I suggest the original estimates of 1% for each of the factors were more a 'try for fit' rather than being based on any empirical data or bottoms up calculation of the probability of life evolving that was the subject of Pebbilta's original question (and is in any case meaningless).
For the opposite side of the coin see the 'Fermi paradox' from the same source at
http://en.wikipedia.org/wiki/Fermi_paradox
Back to my previous comment on the scale of things, there are trillions of stars in the universe at distances we cannot comprehend but, because of the limitations of the speed of light and our technology, we can only observe detailed planetary conditions withiin our solar system, so whilst the chances of us finding intelligent life are small because of this restricted view, the number of intelligent civilisations may be large because of the vastness of the cosmos.
Which shouldn't stop us looking for extraterrestrial life, just best if we don't expend too much time, money and effort given the problems closer to home.