Replicating User Authentication

I have WordPress 2.9 and SMF forum (yes forum forum. so sue me.) and I have linked the databases with a script that makes them always have the same user data in both the WordPress and SMF databases.

Now I need to make it so that 1 login can log a user into both sections of the site. I have figured out how to replicate user authentication in SMF but I can’t seem to figure it out for WordPress.

If someone could give me a push in the right direction that would be great. A tutorial, file name(s), code that they used, anything.

Help is most appreciated

-Commander