Yeah, I mean, nobody's saying that it wasn't Nolan Bushnell who signed it or anything, but you'd run into the same issue if you were to say try selling your system to a pawn shop. Autographed items these days require some kind of documentation to prove authenticity....and just like it was mentioned before, items with signatures of still-alive people are worth less than signatures of dead folks.
And of course it doesn't help any that the signature is on an Atari clone...
But still though, if you do have that picture with Nolan Bushnell, we'd love to see it!