I'm sure its possible. As mentioned, Nikon cameras can do it and I'm sure Canon engineers are just as capable. With Nikon, If you mount a DX lens (similar to EF-S) the camera camera recognises this and works in "crop" mode. Its not ideal as you potentially lose a lot of megapixels. But otherwise, everything works fine.
If Canon, can't get things to work properly, due to mirror sizes, lens element to sensor distances etc then there's an easy solution. The camera just works in mirror-up mode with EF-S lenses. Rather than using the viewfinder, you can autofocus and frame via liveview. Its really not that hard. In fact, this would work really well with the new touch screens. While there are some historical reasons why they weren't compatible, I suspect that some of the innovations seen in the 650D will be seen in the next FF and this will give the camera the data processing capabilty to mount EF-S lenses. If Canon doesn't do this, the decision will be purely sales driven. It forces people to but new lenses if they move from APS-C to FF.