[quote name='Geoff Mendicino']i'd think microsoft is advanced enough to be able to check how things were obtained, if they got it through marketplace or through a code[/QUOTE]
Indeed they are... As a matter of fact, you can check on all your download 'content' yourself:
http://live.xbox.com/en-US/accounts/MyAccount.aspx
And select 'Purchase History'...
Additionally, they also know how the item was 'validated' and, if purchased, how much was paid for it, although this information is not shown on the page mentioned above.
The people that used a code that came in the game case should not be affected.
So, as far as I can tell, the only trick is going to be how to tell the difference between the people who had legit access to download via the web site, and the rest of us. Course, if I understand correctly, this DLC should not have been available via the web at all (i.e. it was restricted to LE releases via the code in the case only), which means ANYONE that downloaded it via the web site should not have it.
Then they will have to patch the game to go into this database and verify if you are allowed to use this DLC each time you play. Anyone who has tried to use DLC on a 2nd console that is not connected to the internet is well aware of the fact that most games already do this 'validation' at launch, so it probably won't be that big of a deal to have the game validate this 'DLC' against a revised list of valid user accounts.
They will not "remove" the DLC from your console (for many reasons), but instead the DLC will simply not show-up/function in-game once the patch is applied.
Course, I am with the conspiracy theorists on this one... The leak was intentional to get more attention to the game, since it was not doing as well as expected. This is after-all the SECOND TIME they mess this exact same thing up... But, that is just my opinion.