lucy24 - 9:16 pm on Feb 16, 2012 (gmt 0)
It doesn't matter if there are pages on your site that aren't on the sitemap, so long as they're linked. Search engines will find them. What does matter is when there are pages on the sitemap that the search engine can't find. (Bing in particular says some ominous things about "not trusting" your sitemap if it's got too many 301s.)
For a forum, you may actually be better off not having a sitemap. It isn't just adding material. You have to keep track of every thread that is moved/ merged/ deleted and adjust the sitemap accordingly. G### will still raise a ruckus every time it can't find something that was there in 2007, but at least they can't claim it's your fault.