found this on MacFixit:
Display Resolutions are Incorrect
Another prominent issue (after installing the 10.5.7 update) seems to be certain display resolutions are disappearing, resulting in the computer outputting non-native display resolutions to the monitor. In most cases, instead of being able to choose 1920x1200 as the resolution (the native resolution for many widescreen displays), the computer will only output 1920x1080 (the maximum for HDMI connections). This seems to be a conflict in the drivers, since 1920x1080 is the maximum display resolution for HDMI connections.
There may be several approaches to fixing this problem. The first would be to try a PRAM reset, which can be done by holding the options-command-P-R keys all at once at reboot, and holding them down until the computer resets a few times, then releasing them and allowing the computer to boot normally.
If a PRAM reset doesn't work, try a workaround suggested by Apple Discussion board user "cizko", which is to install a screen output tweaking program called "SwitchResX" for fine-customization of monitor outputs. The program is a demo, but installing it, setting the screen resolution, and then optionally uninstalling it seems to clear the resolution problem the OS X update.
A word of note: some people have found that in using SwitchResX, the only way they were able to get things to work was by going to the "Timing Parameters" settings and checking "Use Simplified settings", and then changing the drop-down menu from "GTF" to "CTV-RB".
