According to the insiders over at IGN, every copy of the intitial run of Crackdown will contain a Beta Invitation key/code. 

Some were lead to believe that only pre-orders and very limited specially marked boxes contained access to the beta holy land.  So to repeat, it appears that every Crackdown copy (from the intial production run) will contain a unique code to enter the Halo 3 Multiplayer Beta.  Just don't expect to buy a copy (months down the road), as subsequent production runs will not contain a Halo Beta Golden ticket/access code.