This is one of the very few things I miss about Android. Google's music cloud was a fantastic way to access all my music. I hope MS follows up with a similar service.
I was just thinking about this today. I've got a lot of music stored on OneDrive at the moment and although I can play individual files within the app, I'm amazed that there's still no integration with XBM. I should be able to "see" music stored on my OneDrive directly within the XBM app, that's all I want. Nothing fancy or clever, just the basics would be nice.