Aaron Wormus has posted some great historical notes from back in 2002 looking at the state of PHP content management systems, most of which forked off of PHPNuke.
I was just browsing through my OLD advogato profile and thought I would link to these two posts (from mid 2002) which talk about one of the many CMSs which forked off the original PHPNuke code base.
Part one talks about the split between Postnuke and PHPNuke as well as what some of the differences are. Zope is also mentioned.
In part two, he notes a bit of a change in his thoughts on Postnuke (related to a change in the code for a templaing engine).
It's interesting to look back and see how far some projects have come...