It seems the more complex the sentient mind, the slower it evolves. Apparently this is because the genes in the complex mind code for proteins that have complex interaction with other molecules in the body: "change a gene too much and it will be unable to continue its existing functions" (Barone, Par. 2). Thus, the more a brain evolves, the slower its evolution becomes. Though some postulate that the recent information revolution, with its explosion of information and rapid technological change may add extra evolutionary pressure on our brains.
Anyway, this may be another reason why sentient life needs long "boring" (no planet-wide catastrophes) stretches of time to develop, and another reason for some to believe that while life itself may be fairly common in the universe, intelligent life may be somewhat (or even very) rare.
I disagree that it would necessarily be rare - hopefully, if a sentient species makes it to space travel, it will begin colonization and thereby increasing the likelihood of its survival as a species. Space colonization would have an evolutionary pressure on the beings - living in different gravity fields, electromagnetic fields, or atmospheric oxygen compositions for instance, may cause evolutionary changes. These changes wouldn't necessitate brain changes, but body adaptation changes (longer or shorter legs, larger lung capacities) which are not as complex changes to make as are changes in the brain.
Of course, that's skirting around the issue a bit - or cheating: a few sentient species spreading throughout the galaxy is not quite the same thing as sentience being a common result of habitable planets.
One result of this (as complexity of the brain increases, evolution of the brain slows) may be that the old, oft used Sci-Fi adage that advanced aliens would be to us as we are to ants may not hold as much water after all. Sure, a species that's been around for a billion years longer than we have, would have an evolutionary leg up on us, but to compare us to ants? Maybe a lemur would be a more appropriate comparison (by the way, there's an interesting article "Lemurs can be liars, if they think you want their food" in the May 2006 issue of Monitor on Psychology. Not so dumb, after all, these lemurs.). And how many billion year old civilizations would their be? And how many of them had continuously advancement (no dark ages, or no catastrophes that set them back)?
I've wondered sometimes if that would be another reason for extraterrestrials to come visit the Earth: a civilization that was a billion years old may very well have forgotten how they came to be - look at how just much we don't know just several thousands of years back in our own history. They may have great curiosity at how a primitive technological civilization begins. Think how intrigued we'd be if we found a planet of somewhat similar looking creatures were in their stone age era. We'd want to observe them, to maybe get some ideas to help us figure out our own past. Sure, there would be differences, but even the differences can teach us something. It is like studying the weather on Venus and Mars - both have extreme difference in weather, but the two planets are similar enough to Earth in other ways as to be very instructive; Venus and Mars are like two experiments that help us to set parameters, or to see certain forces more clearly on them that are also at work on Earth, but not as obviously.
Barone, Jennifer. "Not So Fast, Einstein." Data. Discover. October 2007. 12. Print.
Dingfelder, S. "Lemurs can be liars, if they think you want their food." Monitor on Psychology. Vol 37, No. 5 (May 2006). 10. Print. Can also be found at <http://www.apa.org/monitor/may06/lemurs.html> (as of 25 November 2007).