According to the insiders over at IGN, every copy of the intitial run of Crackdown disks 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 on the disk to enter the Halo 3 Multiplayer Beta. Just don't expect to buy a copy a few weeks/months down the road after it is released and expect a Halo 3 Beta Key, as subsequent production runs will not contain a Halo Beta Golden ticket/access code.
The cheapest that we have found the Crackdown is $53.99 over at Buy.com. Submit a deal tip if you spot it for cheaper (known/respected retailers only please) and you will be entered in our FREE Xbox 360 Wireless Racing Wheel Contest!
Preorder it: Crackdown $53.99 (w/ Invite to Halo 3 Multiplayer Beta!) 