The introduction of any security barrier in Windows 7 would fundamentally alter the architecture of the operating system. Mitigations such as User Account Control, PatchGuard, driver signing and ASLR have already produced their fair share of pain, related mostly to compatibility problems. Microsoft simply cannot afford to go beyond just evolving the existing security mitigations and setting new protection layers in place. Just don't expect any of the extra tiers to act as impassible barriers, because this won't happen.
Proof of this is the new position of Software Development Engineer in Test offered by the WinSA. “We are seeking a highly technical, self-starting tester and/or pentester to join our engineering team to find security bugs through a variety of means before we ship. You’ll have the opportunity to create, use, and deploy state-of-the-art hacking tools. You’ll investigate new Windows features for security soundness, and scour legacy code for security flaws. You’ll come to see the OS from the perspective of a target, and figure out ways to defend against attacks. Come help us make Windows the most secure operating system in the world!” Microsoft said (emphasis added).