I don't think a tagline would be a good idea. You'll find out what the site is about when you go to the index, there's a long spiel.

Most users won't be directed straight to the forums, they'd see the main site first.

Note: The main site isn't finished yet, but the idea is there.

Edit: I've changed the forum title so it has the tagline throughout the rest of the site, but I'm not altering the logo. Thanks for the feedback.