In Liferay SO checked out and built from trunk on 13, July. When you add more than one category to the forums in Liferay SO, only one is displayed in the center of the forums page. The category also seems to get mangled with the "Related Content" that should be in a separate right hand pane. Instead you should see a list of all of the forum categories (this worked in previous builds). Categories are there and visible in the small left hand navigation pane. This has been this way for a few weeks now.