Hello,
Thanks for all of the insightfull replies.
Regarding the GPL licence, my understanding is the same as the King, in that I would have to provide a way to make the source available.
I think porting to C# would be a big job for sure - one hope is that since they share the same root syntax, and the .NET framework has pretty good coverage for standard C library calls, that it would not be too overwhelming.
The main deterrent for me seems to be the user input. Perhaps I should wait for a year to see how Project Natal works out - maybe I will get the freeform movement after all.
Chomping at the bit to bring some nice deep RPGs to the console...