ACL Plug-ins

Joomla 1.5 also has authentication plug-in's that let you use the username and password off another system.
There are a ton of free User Authentication plug-ins one can use.

See Joomla's extension directory
http://extensions.joomla.org/component/option,com_mtree/task,listcats/cat_id,1921/Itemid,35/

Joomla comes by default with a Gmail, ldap, and openID and example of how to build a plug-in in the your "Joomla_rootpluginsauthentication".

You really don't need to code your own Plug-in.
There are ton of third party extensions out there.