Just a small note that I was able to use the Ardupilot Mega in an experimental setup with FlightGear. I ported the XPlane 0.43 software to Linux C (using autotools). The trouble was finding out that a simple "usleep" wouldn't cut it, but that reading as fast as possible from the incoming data is essential and some steps should be taken to write data at a rate that is just slightly lower than the receiving rate at the other end ( so, 48Hz position updates vs. 50 Hz read updates in GPS_IMU and 48 Hz FG updates vs. an attempted 50 Hz update rate in FG). http://radialmind.blogspot.com/2010/08/flying-with-ardupilot-mega.html
The only thing to look further into is the use of Serial.print and println in the ground station pde's. The GCS_Xplane uses these as well as others, but it looks as if these are not handled in hardware. If anyone from Ardupilot could comment, that'd be nice. Some expected handling here involves managed buffers handed off to some interrupt somewhere that would send it off immediately after the interrupt declared the line to be available...
For the rest, I'm quite pleased that even the stock Cessna of FlightGear can be manoeuvered respectably using Ardupilot. I suspect that with some more 3D waypoints, you could make a not too large trip around some area as well ! .