Hmm.
I did a Google search for this and it seems you need to add the RandRRotation option to your xorg.conf video section:
Section "Device" Identifier "Videocard0" Driver "nvidia" Option "AddARGBGLXVisuals" "True" Option "RandRRotation" "on" EndSection
However, I have little experience with XRandR (which is the extension that allows this), and even less with Nvidia cards. So I'm afraid I can't be of much further help, sorry. :|
Hope that's at least a shove in the right direction.