I was sceptical when I first heard BEagle mention it....
...got out my dvd and looked. There is a 'black smudge' over "Gibson"s shoulder.
It appears to wander over behind the trees on his left.
Could concievably be a dog...
I'll try and do some screen grabs.
EDIT: Here we go, some pics.
http://s1-04.twitpicproxy.com/photos.../439744332.jpg
http://s1-04.twitpicproxy.com/photos.../439744755.jpg
On the far right of the screen, between the trees. When you view the video, it definitely walks like a dog.