Users of NVIDIA's Quadro 4000 workstation graphics card may have a problem with 3D acceleration when updating to OS X 10.6.7. Apparently NVIDIA's display driver was incompatible with OS X 10.6.7, and during the installation process, the updater simply removes it. Without having the right driver the system can't take advantage of all the Quadro's features, and you may end up wondering why some things aren't working.
The good news is that there's an easy fix. Earlier today NVIDIA released a new version of the driver, and you're a quick update away from having everything running normally again.
There is also a report on the NVIDIA forums that suggests Mac OS X will not boot without having the updated driver installed. After speaking with NVIDIA, it seems that this is not the case, and the post was inaccurate -- you shouldn't have any trouble booting after the update.
It is a nice reminder, however, to be careful when you're making changes to machines in a professional setting; it's always a best practice to avoid making any update to a production-critical Mac until the issues are sorted out (and, preferably, after you've tested the update on the lab bench instead of on the front lines). If you can't afford to have a system out of commission, don't hit that update button (a lesson I've learned the hard way).