I just tried this (dynamic view model in CSHTML) and got the same error as your when using an anonymous class, but it worked fine if I created a named class. I searched but haven’t seen this documented anywhere.
// error
return View(new { Foo = 1, Bar = "test" });
// worked
return View(new TestClass { Foo = 1, Bar = "test" });
David Ebbo clarified that you can’t pass an anonymous type into a dynamically-typed view because the anonymous types are compiled as internal. Since the CSHTML view is compiled into a separate assembly, it can’t access the anonymous type’s properties. Due to this forum post, David Ebbo clarified on (Dec 22 2011) that MVC 3 now has direct support for dynamic.