I have a dark side forum (vbulletin) and today ive installed a hack that allows people to register by using a facebook login, its good in the respect it doesnt reveal the users info (email address etc) but yet in can tie in a users profile pic to become an Avatar.
If anyone wants to try it visit www.technodanceaphobic.com and click on the "Connect with Facebook" button on the right hand side.
I can see a big future for this kind of mod once its properly configured and support so im interested in see Bfarbers work when he gets around to it ;) .
EDIT: It also sends notifications off to facebook when a user is sent a PM and gives the option to publish a topic in a users wall posts on facebook etc, it also makes members who are friends on facebook it also makes them friends in the forum too.