I thought I would give an updated answer using Visual Studio 2010 SP1 / NuGet / Scott Hanselman's totally unsupported utility.
- Install MVC3 (w/ nuget)
http://www.asp.net/mvc/mvc3
- Go to "Tools" menu in Visual Studio and select "Library Package Manager" \ "Package Manager Console"
- Once console window appears, change the "Default Project" to be your webforms project.
- Type "Install-Package AddMvc3ToWebForms" (http://nuget.org/List/Packages/AddMvc3ToWebForms)
This will add all necessary dlls, javascript files, web.config setting changes, etc to the project. If everything was successful, you should be able to press F5, navigate to "home" on your website, and see a sample form rendered by mvc: "Welcome to ASP.NET MVC, upgraded with a NuGet package in a totally unsupported way by Hanselman! No warranty!".