I just couldn't resist the temptation to join this discussion. First on the evolution issue, just to be clear- it doesn't explain the origin of life because that is not what it was designed to explain; such an explanation would be within the realm of chemistry. On the habitable planet issue and the odds of intelligent life arising, we have found that life on earth exists pretty much everywhere. From solid rock to arctic ice sheets, if there's an opportunity to exist- a niche to fill- life will fill it. Shelter from radiation, moisture, a few basic elements, and an appropriate temperature are all that are needed to begin the reactions that result in basic organic molecules. This has been observed in a laboratory setting. Once life gets started, its trial and error process of development gives it the capability to go forth into foreign environments and further adapt.
Contrary to AlaskaSU's examples, a lot of the deal breakers he lists do not apply since there are many ways to achieve the conditions I mentioned above. Also, what's to say a group of aliens that discovers us won't just say "next" and move on? Or just ignore/observe? As the detection capabilities of their new toys improve, astronomers are discovering more and more earth-like planets literally by the day, and are also better able to model the locations of habitable zones, which vary with each unique solar system. I haven't read the book AlaskaSU mentions, but if the author is defining habitable zones as fixed distances from stars, and at the same time counting out binaries, those are two titanic blunders. Binary systems DO have habitable zone(s) and in fact, I remember reading an article fairly recently about a gas giant found within one that could have moons harboring life. I would not be surprised if relatively simple, microbial-type life is relatively common. The real question is whether or not life can take the next step from simple to more complex organisms such as the plants and animals we see in everyday life, and then an additional step to sentience. There is really no way to know how or why this can happen; even knowing the history of human civilization and its development, we can only make educated guesses as to how our species became so "brainy" and capable of processing information the way we do.
In summary, I think extraterrestrial life is probably common. BUT it will be in the form of microbes and other non-intelligent forms. There is no way to accurately model the development of sentience since we have only one example to work with and we don't even understand how that happened. There was very complex life on this planet for a VERY long time before anything here took that leap forward, so my opinion is that intelligent is probably pretty rare, even if life itself is relatively common.
