I'm running 8-CURRENT and I use the 180.60 driver too. Xorg seems to have no problems. Relating to xinerama I have this in my logs:
$ grep -i xinera /var/log/Xorg.0.log
(II) Loading extension XINERAMA
(II) Initializing built-in extension XINERAMA
Still I have seen no crashes.
Also is it possible for you to try and compile the vdpinfo
utility which links with the nvidia-vdpau libs ? I think there is some trouble there too.
If you need me to run any more tests, please tell me.