I'm a lot less worried about Martian life infecting Earth than I am the other way 'round. Lifeforms on Earth have been duking it out for a couple of billion years--both against the environment and against each other. I don't have many fears of an Andromeda Strain problem here. We've certainly got plenty of evidence that simple lifeforms remain viable in space for a number of years. We should assume that there are already earth-originated bacteria on Mars. Whether they're doing anything or not is another question entirely. I don't know how NASA came to the view that they wouldn't decontaminate Mars-bound craft. Perhaps we jumped to the no-life-there assumption a bit too quickly. That really doesn't square with the fact that we dove Magellan into Jupiter rather than risk the potential contamination of Europa. However, I don't think there can be any argument that humans on Mars would bring a lot more in the way of contamination than robots. Of course, I'm squarely in the robot camp anyway--what do you expect me to say?