Space colonization is enivitable, but it won't solve our population problems. Everyone should watch this video http://www.youtube.com/watch?v=F-QA2rkpBSY it discusses exponential growth and population. At our current rate we have 50-60 years till we're extremely overpopulated, and if we find a whole new planet and keep our current rate of expansion it will only last for 200 years or so (if it's a planet like mars.) The long term solution is to control birth rates (as increasing death rates will never be popular) so we need to curb unwanted pregnancies and give people a reason to not give birth, maybe make adoption easier or more appealing (maybe give couples a tax cut), as well as marrying but not producing children.
We also have an oncoming energy emergency if anyone hasn't noticed. If our population continues at this growth rate for 30 or so years, we'll be out of both coal and oil and we won't have enough energy even if we use all the known copper in the world to make the necessary amount of solar panels to provide energy for everyone. There is also a growing need for water, there is only so much usable water in the world and people need water to live, if our population grows, that means less water for everyone. The key to all our growing problems is curbing population growth. This will increase the quality of life for everyone who is on Earth.
