I set up my development and deployment environments on Ubuntu with virtualenv with the --no-site-packages option to isolate them from packages in the system installation. My application uses pyrsvg and it is installed by default as a system package. Consequently I had to link the shared libraries it installs (w/in gtk) into my virtualenv.
Here are the links I created (workon and cdsitepackages are virtualenvwrapper shell aliases):
$ workon project $ cdsitepackages $ ln -s /var/lib/python-support/python2.6/gtk-2.0/rsvg.so . $ ln -s /var/lib/python-support/python2.6/gtk-2.0/gobject . $ ln -s /var/lib/python-support/python2.6/gtk-2.0/glib .