If you get a BETA, they will give you a key and you go on marketplace, then "redeem code"
Type in the digits and wahla!
(W@W did it by people who pre-ordered. They gave out keys - but after a while it was for everyone
-Marketplace-)
Dont know whats gonna happen with MW2.. I didnt have a 360 when cod4 was giving out betas - I brought game and console April 2008 :\
PM me and i'll give you more info if ya wish