On a Virtual Machine, I had Visual Studio 2008 SP1 installed along with ASP.NET 3.5 SP1. I tried installing the newly released ASP.NET MVC 2 Release Candidate 2 on that machine and received the following:
The project file '<project path and name>' cannot be opened.
The project type is not supported by this installation.
To resolve this, ensure that the VS 2008 tooling support for MVC 2 is installed in Add/Remove Programs. Regardless, uninstall all MVC 2 installations (should be 2 items in add/remove programs if the previous is the case). Then reboot your machine and reinstall. It should work now.
Hope that helps!