The NASA rovers Spirit and Opportunity are currently attempting to find information that can answer the questions: "Has there been life on Mars", "Is there currently life on Mars", and "Could we some day live on Mars"?
Can there be life on Mars?
The two rovers have found potential evidence of water on our red neighbor. Since so much of life as we know it is dependent on water, it is theoretically possible that life could have existed on Mars. But recent rover findings suggest that Mars had a very hostile weather environment, making it unsuitable for life. Until solid evidence of life is found on Mars, it's currently anyone's guess as to whether life existed there in the past.
It is possible that some day Mars might sustain life in the form of humans. But in order for that to happen, science will need to advance. For example, water would not be easily obtainable on Mars for human consumption. Astronauts likely wouldn't be able to bring enough water along with them, so the need exists for an instrument to convert chemicals in Mars' soil, atmosphere into water (or even astronaut bodily fluid into water).
Travel times to Mars also pose big exploration concerns for humans. The current estimates for a trip from Earth to Mars range anywhere from 120 days up to 260 days — and that's just one way! The temperature on Mars also poses a hazard to human colonization. Bring plenty of winter clothes, because the average surface temperature on Mars is -81°F.
Science and technology have put men on the moon, so it's very possible that there will some day be life on Mars. However, current technologies do not exist to make traveling to Mars (and back) a routine task. One other question that some day needs to be answered: will earthlings that colonize Mars call themselves Martians?