There was a very cool article in Harper's Magazine a while back about the Eastern Panther. April 2004, "Aslan Resurrected"
There are many people in the mountain areas up and down the east cost who have seen evidence of mountain lions, but never any hard proof. The most bizare thing is that the feds, whether EPA, Fish & wildlife, Game, whatever, all deny that they are around. There are a number of people who believe that there is some sort of small conspiracy going on. Some say the feds are secretly reintroducing them, some that the feds want cover it up the fact that they never totally left. The most bizarre thing is that most of the best evidence, like a video tape that one guy sent to F&WL, gets "lost".
There's even a group that's trying to find more proof, composed half of amateur wildlife researchers and half conspiracy guys. The Eastern Puma Research Network, unfortunately they don't have a website.
I don't know if that track is well preserved enough to be hard proof though. The overall shape looks much more like a cat than a dog, but you can't see the three lobe structure in the heel pad.