Comment Re: Have Both (Score 4, Informative) 567
It didn't work for some reason when I had a fairly old ATI/AMD graphics card (It didn't take into account the rotation of the portrait monitor), But when I replaced the card with a mid-range nVidia card the problem went away. My guess is the ATI graphics driver wasn't properly reporting the orientation and pixel layout information received from the monitor.
I have seen some (usually cheap) monitors that don't appear to have an option in their menu to set their orientation. My guess is ClearType probably wouldn't work properly on them since the DDC information would be incorrect when rotated, but that is more of a problem with the monitor than Windows.