I finally found out what the problem was :

virtualenvwrapper.sh is written in BASH and not in Python. So virtualenv is called from a shell (zsh). I didn't have to bother about my PYTHONPATH, but about my PATH (I was already able to import virtualenv from my python shell anyway).

I just added the correct directory to my PATH and everything worked fine.

I finally found out what the problem was :

virtualenvwrapper.sh is written in BASH and not in Python. So virtualenv is called from a shell (zsh). I didn't have to bother about my PYTHONPATH, but about my PATH (I was already able to import virtualenv from my python shell anyway).

I just added the correct directory to my PATH and everything worked fine (the directory containing the virtualenv executable, i-e /Volumes/Data/nfs/zfs-student-3/users/2013_paris/vmonteco/Library/Python/2.7/lib/python/site-packages which isn't included in my PATH by default despite being the directory virtualenv and other pip-installed tools was in).