Flight Simulator X Service Pack 2 previews the new technology of DirectX 10 to enhance environmental effects such as bloom, realistic water, and virtual cockpit shadowing. You must have a DirectX 10 graphics adapter to enable the DirectX 10 features (the check box for DirectX 10 is not visible if you dont have compatible hardware).
Features and improvements include virtual cockpit shadowing, HDR bloom, water enhancements, and efficiency improvements to increase performance in some areas.
To enable DirectX 10 in Flight Simulator X, open the Settings - Graphics dialog box.
News source: fsinsider.com