I never could find a definitive answer, so my comments are conjecture backed by reading the whitespace between the forum post lines. It appears Natty uses a new release of xorg's X server, and Nvidia hasn't released drivers compatible with it. I wouldn't mind so much if that fact had been stated up front, thereby saving me two days of misery trying to make it, nay anything do xinerama across two monitors. But alas, I wasted an entire weekend and wound up back on 10.04 anyway. I even tried the Nouveau driver, and I suppose it has potential, but it ain't ready for use yet. People who take single desktop/dual monitor capability for granted won't give (K)(X)Ubuntu a second chance.

Teach Yourself Electricity and Electronics by Stan Gibilisco.