From what I've read, it would require a hack. I've been trying to figure this out for months, as I grew accustomed to having Compiz-Fusion effects and a smooth OpenGL desktop to work with on my old dual-monitor setup with nVidia Twinview. Now I'm forced to use Xinerama with my 3-monitor setup, and I'm unable to enjoy Compiz-Fusion.
It's driving me nuts. The more this issue comes up, the more likely it is that someone will develop improved support for multiple monitors. I recommend posting on the Ubuntu and nVidia forums, and if there is a Xinerama forum, there too.