Ah, silly me. Of course Xorg.1.log is the proper one. It's attached and looks clean. I am thinking now this is not a NVIDIA-issue. I thought the error in Xorg.0.log was the cause, but it seems compiz-fusion is the troublemaker. Still it's odd I get this when starting compiz:
Checking for Xgl: not present.
xset q doesn't reveal the location of the log file. Using fallback /var/log/Xorg.0.log
No whitelisted driver found
aborting and using fallback: /usr/bin/metacity
The "nvidia" driver IS whitelisted. But Xgl should be present if it is loaded, right?
EDIT: Still puzzled about the missing whitelisted driver, but disabling compositing in metacity made Compiz work. Apologies, it was not an NVIDIA issue.