|MadSci Network: Microbiology|
Many different kinds of scientists are studying this very topic including geologists, biologists and planetary scientists. In fact, there is brand new area of science called Astrobiology which is devoted to studying the formation and evolution of life on Earth and whether we would expect life to have formed and thrive on other planets.
The immediate answer to your question is that we simply don't know yet which is why so much effort is going into answering the question. The Viking and recent Pathfinder missions did not find any obvious signs of life on Mars and there have not been any official space missions to see if Earth bacteria would grow on Mars because we do not want to contaminate the planet. In fact, there has been some controversy about sending so many space probes to Mars because we could accidently contaminate it with our bacteria. Scientists take extra special care to make sure the space probes are very, very clean before sending them to Mars.
Mars has many properties which are similar to Earth like seasons and dust storms. An important factor for life is water which Mars once had but is now all in frozen form in the polar ice caps and beneath the surface. Mars is quite cold and dry, conditions which we try to reproduce on Earth in various science experiments (see web links). However, many bacteria are able to survive in extreme conditions so they may very well survive on Mars. In conclusion, the answer is something to look out for during the many upcoming Mars missions. Maybe you could help us find the answer!
Here are some web sites which might be interesting to you. http://www.seti-inst.edu/science/litu/ps.html
And a great series of novels on this very topic are called Red Mars, Blue Mars and Green Mars by Kim Stanley Robinson. Many other novels have been written about colonizing Mars. Enjoy!
Try the links in the MadSci Library for more information on Microbiology.