I find it's interesting very much.
When the Orchard resolution is opened in Visual Studio 2010, there are many red line in its Them project's views code which remind "The type or namespace name 'Orchard' could not be found (are you missing a using directive or an assembly reference?)".
So I add reference for Orchard.Framework and Orchard.Core. The warning or error disappeared after rebuilding the project.
But what confuse me is why the project still can be built successfully even it do NOT include the two above project?