So NASA scientists are now claiming that the soil on Mars that's been analyzed by the Phoenix lander is capable of supporting life.
If that's true, the next obvious step seems to be to send life to Mars in order to measure its sustainability over time. The article mentions asparagus, but that sounds a little ambitious. However, I'd fully support a mission that attempted to seed the Martian surface with bacteria, which live in virtually every environment, some especially harsh.
I didn't make it through the sci-fi novel Spin, but the plot revolved around a shield that was placed around the Earth by someone or something that made time pass much more slowly relative to outside the shield. Someone got the bright idea to fire rockets to Mars and try to get life jump-started as a way of terraforming it.
Millions of years are not passing on Mars for each day on Earth, but the idea is still a good one. It would be very useful to know how long bacteria could last on the Martian surface, and whether or not they could gain a foothold by being adaptive enough to extract what they need from the environment.
Scientists recently found bacteria that had survived for 120,000 years in a glacier in Greenland, about 2 miles below the surface. Those guys might be a good candidate for the first sustainable Martian colonists.