1. A stylus is only an input device - obviously it can be used to provide input to a
Natural User Intereface[
^], and some people fill find it more natural than a mouse. From the users' perspective it will not be as "natural" as a multi-touch enabled surface, but I would rather create a sketch using a stylus then a mouse.
2. Seems like
The Microsoft Surface Blog[
^] provides a lot of information you will find useful. Including demos based on WPF. Surface provides a more comprehensive API for developing natural user interefaces.
3. With the release of Surface 2.0 SDK you can target any touch enbled device running Windows 7. Windows 7 Mobile devices, and possibly other devices too
Regards
Espen Harlinn