I believe there is an example plugin that might be of help http://www.zenphoto.org/news/zenphoto-single-sign-on.
In addition there is a plugin for Zenphoto 1.4.2 that will enable Zenphoto to use HTTP authorization so your users could sign on via HTTP authorizaton and it would stick in Zenphoto.
The forum software creates links automatically from urls written and sometimes this happens... The correct link:
http://www.zenphoto.org/news/zenphoto-single-sign-on
btw, vincent, you seem to double post today a lot. Any issues with the site?)