Comment 49 for bug 296167

I'm also having this issue, pretty much as everyone explains it above.

Two NVidia cards, 3 cards.
NVS290 - Center and Right
GeForce 8800 - Left

Twinview won't work with 3 or more screens. nvidia-settings might give you a config that lets you login, but don't count on it being usable. If you hand tune an xorg.conf you can get it looking ok, but the window manager doesn't get any of the monitor boundary info so windows pop between monitors, or sometimes off the screen.

I'll give downgrading the xorg packages a chance tonight, but my next stop will be back to hardy.