Well, I needed to do this because the project that the codgen tool in Orchard creates is not an MVC3 project. Call me picky, but I like the Add Control, Goto View, Create View, and all the other cool little things that come with an MVC project in Visual Studio. So, I set out to figure out how to do it. It's pretty easy, in the .csproj file, there is a project type GUIDs element. Starts out as something like this:

<ProjectTypeGuids>{fae04ec0-301f-11d3-bf4b-00c04f79efbc}</ProjectTypeGuids>

just change it to:

<ProjectTypeGuids>{E53F8FEA-EAE0-44A6-8774-FFD645390401};{349c5851-65df-11da-9384-00065b846f21};{fae04ec0-301f-11d3-bf4b-00c04f79efbc}</ProjectTypeGuids>

Of course, you will need to make sure you have the MVC3 stuff installed.