Flight Simulator X Service Pack 2 addresses these post-release customer concerns:
Multiplayer compatibility with Flight Simulator X: Acceleration
DirectX 10 preview

IMPORTANT: If you have already installed Flight Simulator X: Acceleration expansion pack, do not install Service Pack 2.

IMPORTANT: You must install Service Pack 1 before installing Service Pack 2.

Notes:
Service Pack 2 does not include the code updates required to use the new Flight Simulator X: Acceleration aircraft. You must purchase and install Flight Simulator X: Acceleration to use those aircraft.
After you have installed Service Pack 2, you will be able to play multiplayer games only with users who have installed either Service Pack 2 or Flight SimulatorX: Acceleration.