http://www.newscientist.com/article.ns?id=dn7454
"Terrestrial microbes that hitch a ride to Mars on spacecraft may be able to survive under special circumstances, according to a new laboratory study. The research suggests scientists should take extra care when analysing potential signs of life during future missions to the Red Planet."

...Duh?

I was pretty sure people were already working under that assumption.

Duh!, then. :)