WinRT and build-in Windows 8 Apps
I asked this during the 'Ask the Experts' session at //Build/. The answer was "Yes, within reason. Not all of the WinRT api's will function when called from desktop apps - for example any metro specific ones will fail. No explicit list of which ones work from desktop (non-metro) apps is available."
Edit:It appears that the documentation has been updated to provide a list of WinRT api's available from the desktop.
From "Dev Center - Metro style apps"
A sticky posted by moderator on Thursday, November 03, 2011 1:16 AM
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.