This little mix tape sort of thing has just been released with 2011 Mash-ups
One of them is Dreams and 'Till The World Ends by Britney Spears (aptly titled, Till The Dream Ends).
I actually rather like it. It's basically the vocals of Dreams over the backing track of Till The World Ends.
http://djlobsterdust.com/index.php/m...s-2011-mashed/