I too am attempting to use the Adafruit motor shield with the Netduino. As previously mentioned in this thread, there is not an easy way to control all of the channels on this shield.
I have gone ahead and written up a library for the shield anyways, which is attached. The library is intended to drive DC motors on M3/M4, or one stepper on the pair. Also in the attached zip file is a little test app that attempts to run a DC motor attached to the header marked M4.
As a bit of a disclaimer, this code is basically un-tested. I've looked at it with a logic analyzer, and it seems to be right, but I have not yet tried it with a motor. If you are interested in tinkering with it, please feel free to do so.