Hey Humus, or anyone else involved in that GF3 z-occlusion thread the other day, there is a interesting article on Anandtech.com about various features of the GF3 that have a shroud of mystery cast over them since Nvidia classifies them as trade secrets.
The author makes some educated guesses on how some of it(including z-occlusion) is done. Interestingly enough, he thinks a per-pixel process is being used in the Z-testing and makes guesses to whether the process is on-chip or off-chip considering speed/data limit tradeoffs.
I know this topic is not specifically OpenGL related, but it is very interesting and at least a few people were in a focused discussion about it.
Funk.