I was trying to check the main Orbiter forum today, and for some reason it was down. Checking the forum list on the Orbiter main web site, I found a link to an Orbiter forum I hadn't noticed before, orbiter-forum.com, so I checked it out.
It's really quite good, with a modern and friendly forum interface, and it seems to be quite active. Looking back through the posts, I found it was started in October 2007 by "Tex," an Orbiter user known for his excellent Orbiter videos. Thanks Tex!