If you have an Nvidea card and you have installed the Nvidea proprietary drivers (nvidea-settings), you will notice that you can't rotate your monitor using the Ubuntu settings any more. The solution is simple. Under the "Devices" section in the xorg.conf file found at /etc/X11/xorg.conf, simply add the line
Option "Rotate" "CCW"
where "CCW" stands for counter-clockwise. Naturally, "CW" is the other option.
I am currently working as a Wireless Technologies Researcher at DOCOMO Euro-Labs. My research is focused on interference and energy-efficiency analysis of next generation heterogeneous cellular networks.