Any chance you could add a CSS class to the body element of all Admin pages? This would make skinning pages with complex layouts a lot easier. For instance, I have a skin which has a header image that is extremely out of place on the PageLayout and other non-cms Admin pages.
As an alternative solution, maybe a setting that allows one to set a different skin for admin pages. This setting could be hidden by a web.config key to keep it from being accidentally changed.