I have talked to Miguel about Mono on mobile devices.   It would be difficult.  Of course there is AppForge (http://www.appforge.com/index.asp) that works on BlackBerry, Palm, Windows Mobile and some Sony, Nokia and Motorola devices.  But now .NET CF has been ported to Symbian (http://www.windowsfordevices.com/news/NS3653096759.html).