Server crash with X.org 7.3 and Xinerama
X.org 7.3 enables the Composite extension by default, but it fails to initialize when Xinerama is enabled. This causes a bad interaction with the NVIDIA driver resulting in a server crash. To work around this problem run
# nvidia-xconfig --no-composite
to explicitly disable the Composite extension.
Mouse problem with X.org 7.3 and multiple X screens or Xinerama
The version of xf86-input-mouse shipped with X.org 7.3 contains a bug that confines the pointer to one of the screens. You can avoid this problem by using the xf86-input-evdev driver instead.
[Edit: This is fixed in xf86-input-mouse-1.2.3]