Intelligence is no different than feathers or tentacles or petals. It's a biological trait with both costs and benefits. It costs energy (the calories we use to build and run our brains) which we could otherwise use to keep our bodies warm, to build extra muscle, to ward off diseases [...] it is by no means a given that intelligence is always a net plus.
He goes on to note that we can breed fruit-flies to be better at learning, but then they do worse in competition than the dumber ones.
PZ Myers explains his pessimism regarding the search for extra-terrestrial intelligence:
[T]here doesn’t seem to be any evidence for a predisposition to favor intelligence in biology. Features like multicellularity, photoreception, long sharp fangs, flight, etc., pop up in life’s history over and over again, independently; but intelligence? Feh. The universe doesn’t seem to like smart guys. We happened once, and what’s more, we seem to be teetering at the end of one long chain of improbable events in the history of one marginal set of lineages, of which most of its members are in decline.
I admit to having some fondness for intelligence, so it's disappointing to learn that there probably isn't as much of it in the universe as I had hoped. Though we only have one planet's worth of data here to analyze. Perhaps this is really a question for synthetic biology to answer?
PZ also lists a few other reasons why we shouldn't be surprised by the lack of aliens on our doorstep. My favourite:
Once an intelligence has ensconced itself in a comfortable shell of civilization, there’s no further incentive to be smarter, and there’s even pressure to be less clever and fit in. Maybe civilizations reach that point where they invent TV, and then everything goes downhill.
PZ's post is also picked up by Brandon at Siris, who makes a very interesting point:
[W]e have fairly good reason to doubt that, even on the questionable assumption that there are lots and lots of intelligent species, most of them would ever reach considerable technological advancement. An immense amount of our scientific development, for instance, is tied to size of our moon. Who knows how we would have developed scientifically and technologically if we never experienced total eclipses? [...] I think it is sometimes too easily assumed that the history of human intelligence shows some special predisposition to science, in a way that is parallel to (and as problematic as, or more problematic than) the assumption that the history of life shows some special predisposition to intelligence.
Lastly, Mixing Memory has a related post on the costs of consciousness specifically:
In a great deal of philosophy, thought and consciousness are synonymous [...] However, most of what we do, and I would even say most of what we "think," is largely unconscious, or at least not under conscious control. While this may seem like a negative, it's actually a pretty good thing. Conscious thought is effortful. Not only does it take up cognitive resources (e.g., attention) which places limits on the number and complexity of the tasks that we can consciously perform, but it also takes up a whole lot of physical resources. This makes conscious thought fairly inefficient. It's fortunate, then, that our brains are designed, as a friend of mine often says, to avoid "thinking" (where thinking is synonymous with consciousness). Most of what goes on in the brain is automatic, relatively effortless, and thus uses a much smaller portion of our cognitive and physical resources.