Last month I reported that ProKit 6.0.1 was breaking Final Cut Studio (primarily DVD Studio Pro) for some users. Well, it would appear that OS X 10.6.6 automatically installs ProKit 6.0.2* which is causing the issues all over again.
Here are some forum threads on the topic:
LAFCPUG: WARNING: Mac OS 10.6.6 UPDATE (ProKit 6.0.2)
LAFCPUG: I think I installed ProKit 601 by mistake - crashing
Apple Discussions: Prokit 5 v 6 Cant use DVDSP and iLife at the same time
Not everyone will suffer problems with ProKit 6.x but if you're one of the unlucky few, follow these steps to downgrade to ProKit 5.1. Note that this will probably break iLife '11.
It is clear that Apple's engineering team is having difficulty working out what the problem is, so if you are experiencing this issue you should file a bug report (requires free developer registration) and give them as much information as possible.
* For some reason I still have ProKit 5.1 after installing OS X 10.6.6. I don't know why this is but I'm not complaining.