Regular readers here on the 'Bot will have noticed my strange (Almost Soviet) obsession with the planet Venus, rather than Mars. It's closer, easier to get to, has more shielding from radiation (Thanks to that atmosphere), and it's got very nearly earth-normal gravity.
I've argued - repeatedly - over the years that we evolved to our environment *including* gravity, and our reproductive abilities simply won't work offworld, or will work disasterously badly. Sure, we can take air, water, food, radiation shielding, and light along with us (Mars is pretty dim), but we can't do squat about local gravity, and that would seem, to me, to be the lynchpin. I've argued on occasion - and written in some of my short storeis - that you simply can't colonize Mars because you can't have babies there. Use it as a retirement home? Sure. A vacation spot? Great! The New West Virginia for thousands of underpaid miners hauling up resources? Swell!
But a self-perpetuating colony? No.
Many have pooh-poohed my pessimism on this subject, but it turns out I was pretty definitely right: http://www.msnbc.msn.com/id/37063832/ns/technology_and_science-space/