> But do you process 'import' lines in the .pth files?

Yes.

  glyph@illidan:~$ echo 'import sys; print "hello .pth"' >
.local/lib/python2.5/site-packages/hello.pth
  glyph@illidan:~$ python -c ''
  hello .pth

> It also looks like you may not have PYTHONPATH set at the time of installation.

I do.  The shell log I entered previously didn't omit any intermediary setup
steps.  PYTHONPATH doesn't contain the installation target directory though;
just the directory containing my bootstrapping sitecustomize.