I'll admit, when I saw the rock on NASA's website I was a bit surprised. But here's the rub:
NASA has produced thousands of pictures of the surface of Mars just from Curiosity alone, not to mention Opportunity and Spirit (may it rest in peace). As curious as the rock is, it would take a whole lot more than a few shadows to convince me that all of the thousands of images are fake.
Sadly, MSL has long since moved on from that place, so no second picture of the same rock will be available until a) NASA diverts MSL from its intended destination just to snap another picture; b) NASA sends another probe to the same crater; c) humans to go Mars and can bounce around and take a picture of the rock. However, considering how the infamous Face on Mars turned out, and the anatomical anomalies have been indicated, it's safe to say this is a rock.
As for NASA faking the Martian photography, where would they do it? The Grand Canyon certainly doesn't look like Gale Crater in any of the panoramas provided. The Earth isn't that big, and Curiosity (and Spirit, Opportunity, Phoenix, etc) have taken many thousands of photographs and to date none have been matched with Earth terrain.
Maybe I'm an indoctrinated fool, but having seen thousands of pictures of Mars (and tens of thousands from NASA's other missions), a squirrel-esq rock isn't nearly enough evidence to discredit them.