Considering the current rate of development within countries, there will be a significant increase in the population, but not enough to totally eradicate the planet. Currently we could fit the whole of the world, 7 billion people, on to the Isle of Skye in Scotland, a small island with very little space. There is a lot of our planet which can by terraformed and yet is still out there to be exploited, like... Antarctica?
Research has shown recently that many countries populations and fertility rates are slowly plateuing, with this, no over population in the near future will be apparent, in fact, if this rate of growth continues, we will kill ourselves before the population becomes to high, mainly due to disease and starvation, but at this current time, very little is hapenning. Lack of resources maybe? Not particularly, with the amount of food being processed by the world, it is easily enough to feed everyone, the only reason there is famine, is corruption and greed and human ignorance, and with that, human ignorance would compromise a lot on Mars, such as policies to protect and own land. It's hapenning in Antarctica, so will Mars be any different?
We have new pioneering science and technology to improve world energy supplies, currently in Scotland wave technology can provide small towns with vast energy supplies from one single unit, use of this in LEDC countries such as Nigeria or India will give those less fortunate something that they can help lower their death rates and birth rates with better healthcare etc.
Back on topic a little bit, the world will end, in hmmmm lets see, 5 billion years time (or Dec 2012, 50:50 chance). That is a looooooooooooooooong time to wait, a long time to evolve, a long time to adapt. perhaps in the future Mars and other planets will be in our reach, but really to colonise Mars would be more scientific, not a civilisation, but research points for life and extra terrestrial life, like in, well... Antarctica. Perhaps if we want to survive the end, lets move out of our solar system? This will be inpossible in my lifetime, and many more to come, but perhaps we can do it, and Mars will be the first step in a huge journey for human kind.
For those who don't want to read 
We could go to Mars, but in the current climate and financial situation, what is the point other than to expand our knowledge. Thing is, there is still so much to discover about ourselves and our planet. Lets sort our own world out, before seeking others to plunder, destroy and then leave... I agree that it should be done, but not now, later, I want to enjoy this world before discovering another.
Toodles
Torty
