I've been toying with this idea myself for a while, I guess because it's a natural extension of protected mode and helping backwards compatibility in a way that doesn't foul-up a proper 'pure' OS design. Except I didn't invoke a display separation layer, because that would really make performance (and the quality of the user-experience) suffer. The DWM has shown that you can do display surface redirection without any issues with application compatibility, so I really don't think it's necessary.