Multi-monitor gaming systems are becoming relatively popular right now. Adding support for these systems is rather easy to do by (letting the user) limit the HUD and menu screens to a certain width/height and add an X/Y offset to any HUD/menu elements on the screen.
Additionally: FOV in-game should be calculated using the vertical component instead of horizontal component. (A user settable FOV would obviously be superior, as long as it isn't limited to what is considered 'sane' for single-monitor setups! Otherwise the ultra-wide crowd would still be left out.