As Nao mentioned, this was a communication mix up with Xbox - the song was planned to go live today but since we moved the Bass DLC up we had to shuffle the planned song DLC's. The A7X song was only up for a little bit before it was taken down.
If you managed to get the song before it was removed, we believe you'll just have it early! Congrats you assertive fan you!
We can definitely confirm that A7X is coming to the game soon. Sorry for the confusion!
Let me know after you try to download it, we believe you will still get the song.
I'll be able to check when I get home. I have my 360 set to download purchases in the background, so I should have gotten it pushed down when I purchased it. We'll see. It's definitely been bought, I just don't know if it will already be on the machine or not.