Mars exploration, a contamination?

What if someone either intentionally or unintentionally seeded Mars with life from Earth?  Does it matter, if the intention is to eventually colonize Mars?  Understandably, researchers would like to discover life, not seed it.  But at some point in the future, as China, Russia and others get into the business of exploring Mars, opportunities for slip ups will increase.

This brings us to Space.com's article informing us that the recently launched Mars Rover - Curiosity - may have been contaminated, though the risk was minimal.  According to Space.com writer Leonard David, a set of drill bits were tested outside of previously set protocols.  Because the work was done in a clean room, it is unlikely that any contamination occurred.  However, because the drill bits had not been re-sterilized after the fact, there is no guarantee that contamination did not occur.

Mistakes occur; it may not be possible to stop Earth from contaminating Mars with hitchhikers.