The major players in the mobile navigation market don't work on a port of their famous gps applications for WP7.
Source and full article:
http://wmpoweruser.com/why-are-tomtom-and-alk-copilot-turning-their-back-on-5-million/
Source and full article:
http://wmpoweruser.com/why-are-tomtom-and-alk-copilot-turning-their-back-on-5-million/