Activision has stated recently to 1UP that "If there's a licensed song and someone holds the copyright to it, we'll take it down regardless of whether or not someone complains". That means that if you were looking forward to the day that someone would release a cover of Rick Astley's "Never Gonna Give You Up", it looks like you are going to have to do it yourself.
Honestly, I'm not sure how I feel about this. On the one hand, I can see Activision's view; with all the litigation and BS surrounding copyrighted music these days, Activision is covering their own ass by doing so. On the other, it is pretty lame because I could see some people doing some pretty awesome cover songs. I guess if you want to share your music prowess you will just have to throw your thinking cap on and come up with something original.