The forums will be down in order to apply some updates to the server software that runs them. The main reason is to get to the latest release with the latest security patches. As a result of going to the new release, the look will change slightly and some new features will be added. But, it will still be the same forum you know and love.
The planned outage is for Monday July 20, 2009 from 7:00 to 11:00 AM Eastern time.
Hopefully, they won't be down for the entire 4 hour period.