This MSDN article,  http://msdn.microsoft.com/library/default.asp?url=/library/en-us/mobilesdk5/html/wce51conGPSIntermediateDriverFileRegistrySettings.asp, lists the registry settings that redirect the GPS intermediate driver so it reads NMEA strings from a file instead of from a GPS device.  Very handy if you want to test a GPS-aware application in various parts of the world!

Barry