Mars needs spacecraft!
That's my take on it anyway, because it seems to eat them just as quick as we can build 'em. ABCNEWS.com has an informative article
that discusses several possible explanations for the disappearance of the Mars lander. The article goes into a fair amount of technical detail but manages to explain everything very clearly at the same time.
One interesting fact mentioned in the article was that a failed Titan IV rocket launch in April was apparently caused by two of the rocket stages accidentally being taped together. How'd you like to be the guy with the roll of duct tape on his toolbelt on that one? "Oh, those were stages? I thought it was cracked!"