1920 x 1200 resolution monitor with VGA not detected by Mac OS X 10.7.2

I recently got a Dell 2412m monitor with a screen resolution of 1920 x 1200. But in system preferences it only shows 1920 x 1080. I had it connected, beautifully, to my spare linux machine, so that's not the issue. How can I get the correct resolution on my display? Specs: Dell 2412m Mac OS X 10.7.2 Connecting with MiniDisplay Port to VGA