Do you want to implement a today screen "theme" or have the device operate in kiosk mode? The first one is easy - you would need great experience to do the second one.
There are commercial solutions available that will enable you to operate your device in "Kiosk" mode. These aren't cheap though. If you plan to do it yourself, then good knowledge of Windows CE OS is needed - I suggest that you download Platform Builder (I am not sure how they call it now) and have a look at the source code. This is the best way to learn about the internals of the system.
One, not so perfect but easy to implement solution, is to create a full screen application and have it launched at start up. You need also to intercept the hardware keys.