Just documenting steps to get this set up, so as I don't forget.  Sorry but couldn't find a concise summary to fit my specific desire: do it on linux.      I'll clear this up in the wiki if I could have access to it.

Flightgear works, have that running.  Available in synaptic in ubuntu, easiest way to get it.

Now, need interface program, receive APM serial data outputs (servo commands), and send Flightgear simulation states (fake IMU signals),  both over ftdi serial cable between pc and APM.    It sounds like that Micheal Oborne's neato ArduPilotMegaPlanner program will run on linux, if I have mono. (reading the wiki page http://code.google.com/p/ardupilot-mega/wiki/FlightGear,  and blog http://diydrones.com/profiles/blog/show?id=705844%3ABlogPost%3A1862...)  latest APMPlanner is posted here:   http://code.google.com/p/ardupilot-mega/downloads/detail?name=ArduP...

But I've never heard of mono except in high school.   Oh,  this mono,  http://en.wikipedia.org/wiki/Mono_%28software%29        which runs .net programs on linux.     

So, ubuntu 10.10 already has mono.   ok, no highschool kissing jokes now.

download the APMP .zip file, unpack it, and open a cmd window in that directory, and type

$ mono ArdupilotMegaPlanner.exe

error message to the effect 'assembly mscorlib.dll was not found or could not be loaded.

need something called 'Nant' which includes some required files.  install that:   sudo apt-get install nant

And now it works!  APMPlanner launches.   I'll see now if can get it working together.  Thanks Michael for making it cross platform/OS compatible!

so, short answer, at least for Ubuntu 10.10: 

-unzip it, put it where you want  

-use mono to launch it,   $ mono ArdupilotMegaPlanner.exe

-you might need to install nant,    $ sudo apt-get install nant