I updated Mac OS to 10.5 (Leopard) and confirmed that FlightGear Mac OS X 0.9.10, 0.9.11-pre1, and even CVS/OSG are fully Leopard Compatible. You can enjoy FlightGear Mac OS X on Leopard as well as Tiger.
For those who already installed 0.9.10, make sure you have the latest GUI launcher. If you don't have it, download FlightGear-0.9.10-r114-launcher.dmg from Downloads page.
I test FlightGear Mac OS X only on Leopard updated from Tiger, so reports from the users who have clean-installed Leopard are very welcome!