I run a Halo Info website, and like to give the Facts to my Friends on XBL.
According to this site BNet, you simply need a Copy of ODST to get a Beta Code.
However according to a Friend he found out from a Xbox.com Video that you need to Pre-Order the Game to get it.
So if anyone can supply a solid source that says which is correct, Thanks.
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Check this link: http://www.bungie.net/projects/odst/features.aspx#beta It says you only have to buy the game. It also says that preordering ODST will get you the ability to play as Agt. Johnson in "Firefight" (what I assume to be a game mode in ODST). Since they have a preorder bonus section, I think it's safe to say that the Reach beta has no association with preordering ODST.