Microsoft has started sending them out
Halo 4 takes a decidedly different approach to prestiging in a first person shooter. Normally after reaching max level in multiplayer in a contemporary FPS, you'll have the option to "prestige" and start over. 343 Studios has instead opted to go with "specializations," which net you new armor, and special abilities after leveling them up.
Microsoft has stated that you'll have access to every specialization (all eight, along with the two base at launch) if you buy the Limited Edition of the game, or play Halo 4 online before November 20th. For the latter folk (myself included), codes started going out late last night to your Xbox Live email.
The email is titled "Here is your gift from Xbox LIVE," and it should be from "[email protected]" It seems suspicious, but clinking the link inside (after verifying the email as legit of course, as this is a prime time for scammers to do their thing) brings you to the Xbox.com code redemption website, and allows you to register the unlock.
Just boot up Halo 4 and you're good to go! If you're having issues, check out this thread.