Here's a quick video showing you how to re-arrange Discussion Forums in a Groupsite (for when alphabetical order doesn't quite cut it). 
We also show "drag and drop" instructions below -- which apply only to Groupsites on our "new" discussions application.
How To Reorder Discussion Forums in a Groupsite from Shaun Callahan on Vimeo.
Reordering Forums in our "new app" (May 2012)
Some Groupsites have been migrated to our platform's new application for discussions and do not display the "Reorder Forums" button shown in the above video. However, Groupsite Managers can still reorder forums.
Reordering forums occurs at the main Groupsite level. Login to your Groupsite and go to COMMUNICATE > Discussions. If your Groupsite uses our new discussions application, simply drag-and-drop. "Grab" the crosshair icon located to the right of the forum's title. Drag the forum into the desired order, then release it.
Moving to the new application for discussions is seamless. This means accessing discussion pages on the new app is transparent to any Groupsite user. Currently a small percentage of our Groupsite are on the new application. Over time, all groups will be migrated to the new application. If you are a manager of multiple Groupsites, you may find that some of your Groupsites have a "reorder forums" button, and some do not, for this reason.