Sure enough, Microsoft found a way to make sure people interested in the Halo 3 beta buy copies of Crackdown and not simply rent them...
Looks like the Crackdown disc is not only required to download the H3 beta - it is required to access the beta each time its played.
In fact, according to the card accompanying the game, people must go through Crackdown to play the beta, not the Xbox dashboard like other demos and arcade games. For those unlucky people not choosen for the previous two Halo 3 beta entry contests, actually buying Crackdown really seems to be the only option left.