Oh, that's not Simple/Joomlaboard running the forum at Joomla.org. That's SMF (Simple Machines Forum). Joomlaboard is just the rebranding of Simpleboard.
You can find it at
www.simplemachines.org/
A couple bridges have been developed to bind users in both systems, but none have been shown to work with CB RC2. If you're interested in taking in a Joomla/CB/SMF history long enough to write a book, a few of the forum threads here and at SMF and at the Joomla 3PD forums chronicle it quite thoroughly.
But there's hope. Joomla 1.1 will feature user bots that'll apparently make the task of binding user sessions on two systems a whole lot easier.