The WinFX Sept CTP will not install on 5231 rendering this build of Vista useless for anything other than eye candy.
If you are doing WinFX development with the WinFX September CTP you should stick with Windows Vista 5219 (PDC release) or XP or Server 2003 for now.
Windows Vista 5231 released yesterday to msdn members (and other beta participants) is not compatible with the WinFX September CTP. Unfortunately, we don’t have a matching WinFX build for this build of Windows Vista.
The good news is that we have since fixed this problem, so all future Windows Vista public releases will include WinFX with it.
While bad news from the development point of view, there has been some detailed tire kicking and I expect some compatibility testing from folks who expect to initially support Vista in Win32 compatibility mode.