Here is one possible solution:
Fluid - Windows Mobile .NET Touch Controls[
^]
Another possible solution is to create screenshots of your WPF app, use 9-slice devision (as is commonly done with web pages) and paint your UI using the resulting bitmaps. Not as graceful as SilverLight on Windows Phone 7, but it will help you to create a nice looking UI.
Regards
Espen Harlinn