It turns out that by default, on Leopard, Python 2.5 looks at ~/Library/Python/2.5/site-packages for Python modules. The problem is that nothing installs there by default. However, distutils (and setuptools) will read settings from ~/.pydistutils.cfg allowing you to configure some default settings for the install and easy_install commands. After a little bit of work, here’s the final result:
Now I can worry less about what’s going to happen with my Python modules the next time I upgrade.