Hi Ed, it will. But i don't know at this point how the city will be compatible...
I started Nairobi airport and city initially for FS9. But now, i plan the city for FSX in priority because the phototexture resolution is not good enough in FS9, due to FS9 limitations(4m/pixel). And adding one house after each other is tricky, i wouldn't do the job twice.
For the airport, i used a Gmax polygon textured with a photo texture so the resolution is good enough, even in FS9.
So the airport willl be the same for both FSX and FS9 version. But the city in FS9 may just consist in main CBD buildings.
In FSX, there will be a photoground texture for the whole city.