ok im opening a really old threat on this but in my research i found this site in the top ten pages for what i was looking for... so i figured i would go ahead and post a link containing a PHPBB Forum mod for a sitemap.. http://www.phpbbhacks.com/download/5380 Hope someone finds this useful
If you have the forum integrated with the rest of the website (i.e. not in a subdomain of its own), you could also use a website crawler sitemap creator. It might be slower (after all, a website crawler can not retrieve all the data from a DB), but should otherwise give much the same results.
A website Crawler has the added advantage of seeing your website like the search crawlers do. This may uncover unknown errors in your site that a plugin won't find because it just creates a sitemap of what *should* be there. It wouldn't be good to have Google catch your coding errors and you still don't know they are there. A true web crawler would also be able to find pages of your site that aren't in your forum database, but even if all you have is phpBB, a true spider ought to let you know whether or not you have a spider friendly website and where to look for potential problems.