I have 22" external monitor that has a native resolution of 1680 x 1050. When I open the display preferences I don't get it as an option. I've tried rebooting. That didn't help.
I have the monitor hitched up through a mini-display port VGA dongle.
The problem would appear to be one that has been around for some time, at least two years according to everything I've found for example MacRumors Forum.
SwitchResX provides a way to force a monitor into a mode that the drivers says it doesn't support.