xb - i am pretty sure seo-board comes stock with rel=nofollow on all outgoing links. I think any links posted by admin are regular and links posted by members are rel=nofollow anyway. Now, not all search engines look at this (like Yahoo), but if it's the 'rankings' you are worried about you need not. As far as directing 'users' to other sites, search engines not only look for inbound links from related sources, they also look for outbound links to good information or content sources (like a portal that gets popular by directing you to the right places that are considered reputable by search engines).
I would say, let them post and keep an eye on the link destinations to make sure they are worth sending your users to. If you are worried about getting users to come back offer some bookmarking links near the top so it's easy for them to save in IE and FF.
You should already be posting links to your forum in the others so like the others said, it's only fair! Make sure you dont waste your time posting a link in another forum just for the link pop., they might be using rel=nofollow too! You should post links to your forum when a topic discussed here, answers a question posted in one of the others that is not yet covered there! Just my 2 cents, what you are doing is cool!