Bug #1002641 (“after upgrade to 12.04 amd64 Gnome Classic not properly draw second screen”) seems to be impossible for me to solve: A laptop with Nvidia cannot display on an external display or the laptop display on the same time in Gnome/Unity Ubuntu. You need to switch to KDE; or disable the Nvidia driver and run on the external screen only; or enable the Nvidia driver with TwinView by settings in nvidia-settings (I guess you need to find a screen that has the capability for the same resolution as the laptop screen).
The problem has persisted for over a year in the Ubuntu distribution and questions the applicability of Nvidia for Ubuntu.
I have now several times experienced that the X-interface becomes unresponsive (except for mouse movement), where a hard restart seems to be the only possible way out. There are multiple issues with this proprietary drivers.