Since the introduction of iTunes Plus (the name given to the higher-quality 256 bit DRM-Free music), only EMI has allowed Apple to carry their music in that format, with Warner, Sony, and Universal refusing to do so, instead only offering them to iTunes competitors like Amazon, Napster, etc.
What's changed? Maybe nothing. Or maybe iTunes has just kept growing stronger and is now in a position where they can negotiate the same, consumer friendly deal?
The rumor says we may see the change as soon as tomorrow, December 9th.