I'm using Virtualenv with profit on my development environment with
web.py
simplejson
PyQt4
--no-site--packages
PyQt4
--no-site--packages
PyQt4
PySide
pip
easy_install
It should be enough to create an empty virtualenv and then copy the contents of the .../site-packages/PyQt4
directories into it.
I suggest to install PyQt4 once globally, make a copy of the directory, uninstall it and then use this trick to create VEs.