I only post this because working on a client's machine recently I had a new install of Visual Studio 2008 that just disappeared whenever I tried to edit XAML files or even XSD files. This was due to a known problem with the Visual Studio editor system and some time back a hotfix was issued for the problem (KB968760).
It happens that this hotfix has been superceded several times now and in order to fix the problem Mark Wilson-Thomas from Microsoft kindly provided be with the link to the hotfix that worked. I now have my VS 2008 up and running again. If you need the fix it can be obtained from here