Question: How can I know in a C#-Application, in which direction the screen of the mobile device is orientated? (i.e. horizontal or vertical).

Answer: 
In Microsoft.WindowsMobile.Status there is a class which keeps track of all kinds of properties of your device. 
Besides the one you need, DisplayRotation, it also contains properties about phone coverage, Nr of missed calls, next appointment and many more. See [msdn](http://msdn.microsoft.com/en-us/library/microsoft.windowsmobile.status.systemproperty.aspx) for more info. 

You can also add an event-handler to be notified of changes of these properties.
Add a reference to Microsoft.WindowsCE.Forms to your project. Then you can reference the 
Microsoft.WindowsCE.Forms.**SystemSettings.ScreenOrientation** property, which will give you what you need.

Incidentally, you can set this property, so it can be used to set your screen orientation also.