And doesn't this plugin kind of achieve this? Rather than marking sites which should be rendered in IE6, you mark them to be rendered in Chrome. So all legacy sites work without modification (using the IE6 engine), whilst modern sites can add the chrome meta tag and get rendered by Chrome.
If we could tag certain sites as legacy, we could uninstall IE6 and default users to Chrome/Firefox/whatever. Or, we could even install IE8 and use the Chrome plug-in for true IE6 compatibility -- the opposite of what Google proposes.
You could simply use the IEView plugin in Firefox to achieve the effect you are looking for. I have a few IE-only sites and it seamlessly uses IE to render those sites while in Firefox. You configure which sites automatically use that rendering engine.
