I just purchased a 24" display to go with my MacBook Pro (Summer 2009 model).
I cannot get the display to go any higher than 1440 X 900. I randomly shut my MacBook Pro cover last night and the screen changed to 1920 X 1200 so I thought it could not handle both displays at the same time. However, when I tried this again this morning the display is back to 1440 X 900. I talked to Applecare and they told me I needed to take it in, but I wanted to see if there are any suggestions.
I have gone into display settings and changed it to 1920 X 1200, but underneath all the options it says "usable resolution 1440 X 900".
There is no issue with it. I have exactly the same set up. You have to close the lid for the graphic card to display at 1920 by 1200. But I have a wireless keyboard and trackpad so I woke the computer up with it and the computer is running with the external display running at 1920 by 1200.