I just found out about this from someone on a seperate forum. For anyone who is concerned that they won't get a Beta Key from COD.com, preordering, etc., you can sign up here.
http://www.gamekult.com/tout/betacod5.html
The site is in French, but it's fairly straightforward.
1. You just need to fill out the first couple fields and last three:
Nom (Name)
Prénom (Surname)
You can skip the middle section labeled "Communauté".
Fill out the following:
Adresse email (Email Address)
Vérification email (Repeat Email Address)
Date de naissance (Date of Birth)
2. Click the confirmation button.
3. Check your email to confirm your email address.
4. Profit.
The confirmation page roughly translates to "Welcome to the enrollment program to play the beta test for Call of Duty: World at War, in multiplayer mode on Xbox Live.
You are registered to receive a key to the beta of Call of Duty World At War on Xbox Live."
You should get email saying
Hello, You wish to participate in the Beta Call Of Duty World At War. You have 48 hours to validate your email by going to the address below or by copying the link in your browser.
http://www.gamekult.com/tout/betacod5.html?action=2&email
You will be registered to receive a key for the beta.