Zope 3.3 doesn't work well with the python 2.4.4 that comes with Ubuntu Edgy. Something about a readline change somewhere in the python standard library. So I had to build python 2.4.3 from source. Being an impatient being, I haven't give it much thought and just unpacked the python tarball, hit ./configure and make install. I ended up with python installed in /usr/local/bin/python, which would be really really cool if Ubuntu wouldn't depend on python and wouldn't place /usr/local/bin at the beginning of $PATH. The problem became apparent when a simple upgrade failed because of a missing xdg python package.
I changed the PATH and erased /usr/local/bin from it, restarted the update and everything was fine. Searching for a bit in /etc revealed a login.defs file that contains the PATH as it would be defined for the users. I changed this file, but I don't have the patience to update now, so I presume that on my next update /usr/local/bin would be moved to the back of the $PATH, as I have specified in it.