If you can structure entire OS's into tabs using virtualization, I can't imagine it being any more difficult (and quite a bit less) to create a generic tabbing window that you can open individual apps within. It may not be the most efficient or best way to do it, but it would certainly fulfill the need and is exactly what I think Kalos was after. Windows already enforces the one app per window, so you would just need to add a tab-like extention that can be yet another view just like tile, full screen, or cascade. Not saying it is easy, but certainly not as hard as you seem to make it out to be either.