The accepted answer was “useful” but after installing MVC4 beta today, a few of my MVC 3 projects would not compile. (ModelClientValidationRule conflict) The fix was:
Edit:
ProjectName.csproj
Change
<Reference Include="System.Web.WebPages"/>
To
<Reference Include="System.Web.WebPages, Version=1.0.0.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35, processorArchitecture=MSIL "/>