having the same problem with drivers up to 185.13. no way to get rid of tearing with twinview. disabling either one display and tearing vanishes on the other (mplayer -vo gl and opengl vsync in nvidia-settings.).
well, got used to it. since when i watch video i usually do not need the other display
of course that is no real fix...
8800gts, kde4 or xfce