The forum is something that Ive thought about. The problem is there's no forum system designed to be integrated into an existing system, the closest thing is Drupal, which lets you integrate your system into an existing one.
This means that phpDip either needs to become a Drupal module (something I have considered), or come up with its own forum system, but a forum system isn't something that hasn't been done before, and it's not Diplomacy related, so it does seem like a waste of time.
But at the moment we may have gained quite a lot of users, but not enough to justify a full forum system, so I think for now some minor adjustments can be made to accommodate the current user base, putting off (or permanently resolving) the need to upgrade to a large forum system.
I'm not sure about having multiple tabs though. Personally I wouldn't want to click 5 times to check for new posts in each category, and we don't get enough new posts to justify that. (Whether that's actually /because of/ the poor forum system is a tricky question, but anyway..)
It seems like what's needed is just a way to make things a little tighter. Instead of having a full post, containing the first item in the thread, the forum should only list the titles, users, times, replies, etc. And instead of a large gap between posts posts would be squashed together.
This way I think all the new posts from a certain day would fit within single screen, and that's a fine compromise.
Editing posts requires a whole new UI function, so I don't think that'd be worth it. A "preview" button wouldn't need anything new though, so that could be added instead.
Also I think private messages were mentioned. Again the problem is that this needs new UI functionality, and I don't think it'd be worth it. With the forum system I really want to keep it simple, and if it turns out that it can't be kept simple any longer then I'd rather go to any lengths to use an existing forum system than write a new one.
Bottom line: I think all that's needed is for forum posts to be crunched up together. Even on popular forums the "new posts" list rarely goes over one screen, and I think "new posts" are all most people are interested in.
Making things more space-efficient, adding pagination, possibly a preview button, and making the screen go down to the last viewed post when you click "open thread", are the only changes on the todo list this time around.
If things expand more (and I don't think they'll expand much more) then some other functionality may be needed.