Tahoe ticket http://allmydata.org/trac/tahoe/ticket/229 -- describes a
problem that I found on my home system, running debian/sid, when upgrading
from the debian python-setuptools-0.6c7 to 0.6c8 . I have Nevow installed on
this box, in a way that is visible to setuptools: specifically {{{import
pkg_resources; pkg_resources.require("Nevow")}}} succeeds.

However, with 0.6c8 installed, "easy_install Nevow" nevertheless attempts to
download and install the latest version according to pypi. 0.6c7 did not: it
saw the existing Nevow and refrained from downloading a new copy.

I haven't done a lot of analysis of this yet. When Zooko and I looked at it
about a month ago, we concluded that it was something specific to the debian
0.6c8 package: installing 0.6c8 from source on a different system did not
seem to exhibit the bug. Downgrading to the debian 0.6c7 package resolved the
problem.

This has been reported to Debian:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=475440

I believe that it is a bug specific to the Debian package of setuptools 0.6c8.
It doesn't happen with setuptools 0.6c8 on other platforms, and it doesn't
happen with the Debian package of setuptools 0.6c7.