This is one of the items that is tested in your Windows 8 App when you use the WACK.
If you are in Southern California, please email this post for more information on getting a token for your app by June 30, 2012!
If your application requires that users sign-in to enable certain functionality, pay attention to the user experience and workflow around sign-in and sign-out.
Try to keep your application functional while the user is still signed-out. Provide some default experience that still has value to the user.
If sign-out is not a common action (such as caching credentials), implement the functionality via the settings charm; this way it will not be intrusive, and the user will intuitively find it consistent with other Windows 8 Metro style apps.