No, OS X does that. The ACD can easily display 1920x1200. I've done it in Windows loads and I can set it to that in OS X too. Don't know why I've got it and nobody else apparently has, maybe its to do with the fact that I have an HD TV attached to my Mac too.
Does it display that when attached to a single link card? OS X did not show me any option higher than 1280x800 for any single link DVI connection to the 30". I've only seen 1920x1200 as an option when it's attached to a dual link DVI connection.