http://photographytoday.net/forums/feed.php
This shows the last 25 forum posts, although it excludes certain categories. The reason that it excludes certain categories is because I use this feed in a couple of different areas, and there's no need to display certain topics, such as this one, for example.
However, it does display all of the photography-related topics.
phpBB uses an ATOM feed, rather than a true RSS feed. Note that you don't need to be logged in to view this feed, but of course you would need to be logged in to reply.
You can even add this to your Wordpress site, for example, by using the URL in a Wordpress Widget (select "RSS" and add the URL).