Bungie will be sending out the invitations to their Beta between January 22 and 26 for those that signed up early. The other means of getting a free beta slot will be starting on February 1 as detailed here.
Of course we all want in on this. Number one, it is free and number two, hands on testing will be a good indicator as to what the feel of the game will be.
If you choose the third option, buying Crackdown to get the code, just be aware that the CD itself is the key. You can’t get a key and just use it.