As for the idea of contaminating something like Mars and having it end up overrun with earth bacteria, I guess it's impossible to prove that it couldn't happen, but I don't it's very likely. Mars is more like the earth than anywhere else in the solar system, but it's still very different. You might be able to find a few organisms here that could potentially survive on Mars, but it's doubtful that any would thrive, particularly to the point of overrunning the planet.
I hope you don't have to eat those words when our giant bat overlords come back down in 30 years.
"Today's robots are very primitive, capable of understanding only a few simple instructions such as 'go left', 'go right', and 'build car'." --John Sladek