The forum not only offers an RSS feed, but also an ATOM feed. It can be accessed by appending "type=atom" to the feed-url.
So the feeds are:
RSS and
ATOMMore information about SMF feeds and their options can be found
in this post.
The ATOM feed has the advantage that it also contains the nickname of the person who wrote a post, whereas the RSS feed would use the email address. This is inconventient since the email address will not be shown for those who have disabled it, and quite often it's not obvious which person belongs to the address. So the recommendation is to use the ATOM feed if you're interested in the names of the posters :)
As for the original posters question, there is no feed for "unread" posts. The function to mark posts as read or unread should be part of your feedreader (some do, some don't). The very notion of "unread" needs some kind of information what in fact you DID read. The usual assumption is that you read everything on each login and nothing when not logged in - thus it shows all new posts since the last login. If you're reading via external feeds the forum doesn't know what you might have read already :)
*purrrr*
