That was my first thought but I am dual-booting to Windows XP Pro and 1440x900 is available in Windows. So if it is the case as you suggest, then how can it work in Windows?
You're not using the windows device drivers in linux however. Please avoid comparing apples to chickens.
I have a monitor similar to this (the Viewsonic VX2025wm) and a graphics card capable of handling the widescreen resolution. The changes I needed to make to /etc/X11/xorg.conf were as follows:
Section "Monitor" Identifier "Monitor0" VendorName "ViewSonic" ModelName "ViewSonic VX2025wm" HorizSync 30.0 - 82.0 VertRefresh 50.0 - 85.0 Option "dpms" EndSection
Section "Screen" Identifier "Screen0" Device "Videocard0" Monitor "Monitor0" DefaultDepth 16 SubSection "Display" Viewport 0 0 Depth 16 Modes "1680x1050" "1280x1024" "1280x960" "1152x864" "1024x768" "800x600" "640x480" EndSubSection EndSection
Note that I'm using an nvidia card with their proprietary drivers, and have no problems at all with this screen. I'm actually considering getting a second one and going dual-head with it.