More than a full (unlikely) hack on Curiosity that would potentially allow to hijack it; I would be interested in finding out how feasiable direct signal de-cryption is. Specifically I can't immagine curiosity having too much of a sophisticated encryption system. Just by knowing a little bit about embedded-systems and hardware encryption, it is probably something like 3DES or AES-256 with a firmware stored symmetric key. This means that the key used to encrypt signals won't change too often and therefore I wonder how hard would it be to break it and get a hold on Curiosity's image and commands feed.
Correct me if I'm wrong, but I'm pretty sure in one of the pictures NASA sent out there were tracks in the ground from some rover moving. Was it not Curiositys tracks? Or an earlier rover?
No other rover has visited this location. So if there was a picture with tracks on the ground it would most likely be from the rover rolling during the landing.
Edit: Just reviewed all the pictures on the NASA MSL page and didn't see any tracks. Maybe a news article mislabeled a picture from another rover?