We have received a number of inquiries in this area. The documentation will be expanding as the Windows 8 project progresses to include more API specific details.
It is possible to use WinRT from Desktop applications. WinRT APIs may be tied to Metro style apps, Desktop apps or potentially available to both. The documentation will list which environments (Desktop, Metro style or both) a given API works in.
Note: Custom WinRT components are only supported in Metro style applications. They are not supported in Desktop applications.
