I wanted to get a hardware in the loop sim running in X plane and I wanted to document my first attempt. I've been able to connect the ardupilot using a binary protocol and a custom Perl socket server based on the work of Jeffery Goff http://www.perl.com/pub/a/2004/07/09/xplane.html
I have everything in place, but a lot of tweaking to do. The really nice part is that there is no extra hardware or soldering to do. The IMU parser will read in the X plane output. The GCS_Xplane.pde when finished will output binary to Xplane. I'll have it working hopefully in the next rev.
If you want to try it and help make it better, I could really use the help. If you've never heard of Perl or Serproxy or Sockets, I don't think I'll be able to help you get it running.
Here is the flow of information
Ardupilot <-> Serial Port <-> Serproxy <-> Perl_AP_glue <-> Xplane