[quote name='twanky']Is there any serial number or key needed to play or download the beta, that's included with Crackdown? If not, couldn't you just rent the game and get the beta that way?[/QUOTE]
No, the disc is the key.
When the beta comes out (if you go the Crackdown route) the option to play it is on Crackdown's main menu, not the demos blade like other betas.