Given Ubuntu already has that functionality built in (Ubuntu One), and offers Win, Android and iOS 3.1+ clients, we're really not so far off from that being the case already.
I run Windows and Debian on my desktops and laptop and I recently decided to try Ubuntu One. I could find no mention of other Linux OSes on the Ubuntu One page, and a quick web search returned nothing encouraging.
I was surprised by this. The impression I got is that Ubuntu One is just for people whose main OS is Ubuntu and who may have to also use Windows from time to time.