Last year, an independent Mars researcher named Joseph Skipper found an object on one of the Mars Rover images that resembled a skull. He posted several version of the image on the web, and speculated about the possibility it was the skull of some unknown humanoid creature. His full story and images can be found here.
Our own take is that this image is quite convincing. It certainly does resemble a skull, with a rounded cranium, two round eye sockets, an ethmoid bone division between the eye sockets, and a significant amount of symmetry. As far as we know, no other pictures of it have been found, but it is certainly intriguing.
It also fits quite nicely into our own Mars Tidal Model. Creatures caught in the catastrophic flooding that would have followed the separation of Mars from its parent planet would have been buried under tons of mud and silt, which would eventually be worn away by the constant winds of Mars, leaving only the preserved, fossilized remnants of what may once have been a vast civilization on Mars. It is this link, more than any other (and described in detail in "Dark Mission") which inclines us to consider that this may be a genuine artifact -- not just of a ancient dead Martian civilization, but of its inhabitants.