OK, you've got a point, MTGox is a central point in a supposedly decentralized network. I think that problem has been recognized, however, and is in the process of being remedied.
Good we're on the same page!
But do you realize that the exchanges as a group CANNOT ever be forked from one another?
I mean hackers could physically try to isolate one from the rest of the network. But after 20 minutes or so the exchange must notice and stop trading before anything is confirmed. At that point it is not a coding problem. It's a humans figure out WTF is happening problem.
No exchange can ever willing decide to be permanently forked from the others.
Agreed?