In the near future this will be supported but for the moment there's no support for it in the Nvidia drivers.
The current version of xfree86 (4.2.0) contains a extension called "XRandR" and this extension can be used for rotating the desktop and for resolution + colour depth switching. (especially that last feature is very interesting ...)
The nvidia drivers don't contain support for this feature. The same for most other drivers I think. This extension was in the beginning meant for PDA linux users.
In Xfree86 5.0 it will be officially supported if everything goes ok. So hopefully nvidia's drivers will have support then too.