You really don't need a screen cover for the D3 or D300 - they have tempered glass. It would almost take a bullet to make a mark on the LCD screen.
Here's a link to a a nutcase on YouTube who does "D3 Torture Tests". In this video he is trying to scratch the LCD screen with a set of keys - no marks. Personally I think he's an idiot for what he does to his gear (drop test, cook it in a microwave, etc), but he does prove his point that the D3 can stand up to just about anything.