Actually. This is really not even needed. IPB3 can do authentication via external db. This allows you to enter : DB/User/Pass/Table Name(and prefix)/username field and password field. Also allows to specify the type of encryption. For this choose, MD5. Make sure you set this to active, Then disable registration.
This is a GREAT method of doing this.
The only thing it does not solve is Single Sign On. I will work on a method for this soon.
ALSO make sure you to do the following to prevent out of sync-ness..
Modify the theme User control panel section for Change password to display a message/link to your change pass page. This is not synched if the user changes pass in the panel on forums. Also make sure you change the Header and either link to your registration page or remove the Registration Part all together. For an example of this check out:
http://efragtv.com/forum