I write mine in ASP so it updates itself. If you've a large forum, you should consider creating a separate sitemap for it as most search engines can only handle about 50,000 URLs per sitemap. Really, the only thing you should have in your sitemap is the first page of each topic as that's the important, relevant content. You should also consider changing the drop down paging in topic.asp to normal links as a lot of bots won't submit forms.
Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”