One of the new features in vBulletin 4 is the ability to use Facebook Connect, and I have just enabled FB connect for the site. There are a couple of features that come from using Facebook Connect:
1) You get the option of publishing new posts or threads to your wall
2) It greatly simplifies the registration process for new users
For existing members, the only real advantage I see right now is the wall posting (and I guess the fact that it lets you auto-import a few pieces of data from Facebook).
For new users, the registration process should be simpler and easier (but we're still running anti-spam software that should prevent this from being abused).
I'll be watching carefully to see what impact adding FB connect adds to the site overall, and whether the quality of new users goes up or down as a result of integration FB connect.