"proj" cannot be opened because it's project type (.csproj) is not supported by this version of the application

If you are using VS2008 pro and get this error it could be you have some un-installed but required components. Re-run an installation while checking to make sure all required components are selected for install.

Also discovered quite by accident, that you'll get the same sort of error...

'C:\Users\Username\Documents\Visual Studio 2010\Projects\ProjectFolder\Projectname\Project.csproj cannot be opened because it's project type (.csproj) is not supported by this version of the application'

...if you are using Visual Basic 2010 Express and do something silly, like try to open a C# solution (.sln file) that was created and saved in Visual C# 2010 Express --- of course!!! The proj file would have been saved with a .csproj extension, and so the .sln file has a reference to a .csproj file.