littleguru said:

My problem is that if you make something like this opt in nobody will actually opt in. Or a small minority. If you enforce it tons of apps break and that version of Windows is going to be even less well received than Vista... 

You could use heuristics to enforce this right now: assume that each directory under %programfiles% containing an executable is an application scope, and monitor filesystem changes made by installers. The monitoring that Application Mode Terminal Services already does is considerably more complicated.