If you wanted forum IDs 5, 10 and 12 as an example it would be: http://forums.digitalpoint.com/external.php?forumids=5,10,12]
I have setup permalinks for everything on my database-driven php pages to hide the get variables - I think it makes it look nicer. At least that was my understanding - maybe I'm wrong, but I dislike variables in any of my links. You have a page to describe your page feeds? As in can I pull a specific thread feed and replies?