zend.ze1_compatibility_mode = On
This line was defaulted in my debian-etch apt-package install of PHP5 and Apache 2.2. However, it caused a ridiculously difficult to track down bug (as it was throwing *no* errors either on screen or in the logs) whereby my install wouldn't even start on my server.
I tracked it down to the second line of code in wp-includes/taxonomy.php, which does some sort of (object) cast. Apparently, the zend-engine-1 compatibility way of doing this causes some sort of silent parse failure (like, no errors or anything kind of stop-failure).
Anyway, this flag is SUPPOSED to just make PHP5 be able to interpret PHP4 code. I would imagine LOTS of people who have PHP5 on their servers, especially web hosts or those who use default package management to install PHP and Apache, probably have it on by default, as I did.
And apparently this didn't affect older versions of WP, as I was able to install 2.0.11 fine on my server. Apparently this bug was only introduced sometime later, at least by this 2.5.1 branch.
It would be nice if the ze_compatibility_mode problem was documented somewhere in the install guides or the "troubleshooting" section, since it is apparently now an incompatible PHP5 setting with WP.