I upgraded an ASP.NET MVC 3 project to MVC 4, and upgraded to VS 2012 at the same time. Everything compiled fine, and worked fine in the browser, but the intellisense was totally broken.
I couldn't for the life of me figure it out. Everything in web.config looked right, the correct assembly versions were referenced, etc.
Finally, I realized that the issue was in the Views\Web.config. The version numbers of assemblies there were pointing to MVC 3 and WebPages 1, rather than MVC 4 and WebPages 2.
To solve the issue, change all the version numbers in that file appropriately (184.108.40.206 for System.Web.Mvc references, and 220.127.116.11 for System.Web.WebPages.Razor references).