I just upgraded from Tiger to Snow-Leopard. I bought the $30 upgrade disk and it worked, no problem.
I am just curious from a theoretical and technical perspective what stops me from giving this disk to friend and letting them upgrade too. (I am not asking about whether someone should. Just if it is possible.)
I don't remember entering a pin number or something like this. I am curious what kind of security apple has in place to prevent you from doing this.