I found a new extension for joomla 1.5 call Jfusion that I"m hoping one of the skilled php coder in the communtiy would be able to setup so our whmcs.com installs could control the login/user details for joomla 1.5 installs.
Here's how it works. The plugin makes the whmcs software database the "master database" for any Joomla authentication. This means that whmcs will be used for all usermanagament such as registration, lost password, permissions, etc.
http://www.jfusion.org is the url
check the development link for some coding instructions.