DLL patching hero Rafael has found a great way to enable your MCE 2005 install to join Windows domains. Rafael, known for patching the now famous uxtheme.dll to enable custom Windows XP msstyle themes has found a reg edit in Windows XP Media Center 2005 to unlock the previously "greyed out" join domain option. This method is pretty complex but confirmed to work.
This will allow you to leave and join domains within Windows Media Center Edition 2005.
Please note: The only other ways to get MCE 2005 onto a domain are to upgrade from a previous MCE or to join domain in setup.
Screenshot: Unpatched Windows Media Center 2005
Screenshot: Patched Windows Media Center 2005