If you are running Windows Vista and have a DirectX 10 graphics card, Service Pack 2 installs a selectable preview mode to show how Flight Simulator will look when the DirectX 10 graphics system is fully enabled. It is likely that this will be fully functional in the next version of Flight Simulator. However, one side effect is that any aircraft that are not DX10-ready may be missing their textures. To resolve this you can simply untick the DX10 Preview in the Display Setting menu.
If you don’t see this option in the Display menu then you don’t have a DX10-ready PC and you won’t experience this side-effect