Try removing all bin/obj directories, and clearing your Temporary ASP.NET Files and %TEMP% directories. Then issue the following commands from a VS2008 command prompt:
ngen /delete System.Web.Mvc
ngen /delete System.Web.Abstractions
ngen update
Also ensure that all your references (MvcContrib, anything else built against MVC) are pointing to the same version of MVC as all the others.
This seems to have worked for me (so far)