I was recently tasked with evaluating Umbraco CMS. When I opened it in Visual Studio 2010 Ultimate, I got the following error...
The project type is not supported by this installation
To fix this, open the CMS UI project file with a text editor (I use Notepad++) and delete whatever you find between <ProjectTypeGuids></ProjectTypeGuids>
Next time you open the solution, all projects will load.
The project file can be found at "Source\Web Apps\Umbraco.CMS.Web.UI\Umbraco.Cms.Web.UI.csproj"