What happens when the Call of Duty 4 - Modern Warfare solo campaign is beaten on a higher difficulty?
Solution 1:
As far as I know, the only difference is (on 360, possibly on PS3 too) there is an achievement for beating the game on one of the top two difficulty settings, and several achievements for beating groups of missions on the top difficulty setting.
Solution 2:
Once you've completed the single-player campaign, you get access to a mission called 'The Mile High Club'. When completed at Veteran level, you get a Achievement that's normally the one you need to get the full 1000/1000 on the XBox version.