I actually just found the actual photo on JPLs site from the spirit archives. Its from SOL 770. Main link below. At the bottom is the photo in question.
This is absolutely the raddest photo I have seen in a long time. Most of the stuff people post you REALLY REALLY have to look hard and use your imagination to see what it is the poster thinks he sees. But this one is just obvious! Do you have a direct link for this from Nasa's site? unbelievable.
That almost appears to be some sort of crustacean - much like a sand crab. Any idea of the size scale it is? I can sort of see eyes on it - and a shell, body and legs.