According to everything I've heard, The expansions [i]are[/i] the unlock keys, that's the point. You don't download the expansion you just download the key and you can play the "expansions" already on the disc.

Instead of basing your beliefs on hearsay, you should probably look into it a little deeper and try applying logical thought processes to it. You don't download the expansion because it hasn't been released yet. There's[i] nothing[/i] to download yet. MS's servers simply have a placeholder key that tells the game and MS themselves that you've paid for it.