I mentioned this on the Creators site as well, but buying songs from XBL before they're officially up is a "do at your own risk" thing. There's always a small chance that a song could be taken down before it ever gets to the actual in-game store. If you bought a song this way and then it gets taken down and put through Peer Review a second time, you would have to buy the song again if you wanted to play it online with friends who buy it after it hits the store officially.
This is because even though the artist/title are the same, it is technically considered a different song as far as XBL and RB2 are concerned. I'm not even sure what would happen if you had two versions of the same song downloaded, or what would happen if you tried to play the song online with people who have the "other" version. It's possible that it might cause the game to crash.