Did the Drake equation thing that Slick posted, used pretty pessimistic figures (by my standards, anyway) and arrived at N=3. Interesting. Additionally, went out and found this:
Mars Colonization Plan
Might be a little on the optimistic side, but it does spell out a fairly serious program. Also, I will admit to be a tad biased in favor of Mars colonization -- I'm a member of the Mars Society.
Mars Society webpage