I spent the whole morning trying to figure out why my Nvidia/Twinview
configuration stopped working.

It appears that I accepted an update download for the kernel (yesterday?)
which put the Nvidia driver out of sync.

Today, two nvidia updates appeared - after downloading them (and undoing
changes I'd made to xorg.conf while trying to debug), my machine was
running twinview correctly.

So - if you're having trouble with nvidia today - check the updates.