I'm experimenting with a forum which lets you switch between threaded and flat format for threads... although I'm a bit worried that, once people actually use it, it might be confusing if group a is browsing in one mode and group b is browsing in another, still I don't know if you have to have one or the other exclusively. The way that the structure of a forum affects the nature of discussion is an interesting problem, though. And probably one that's been thoroughly solved but.. eh.