RSS isn't a standard feature of phpBB3 - surprisingly. I agree it would be a very useful feature, but the mods I've see for this feature look rather clumsy. They normally involve generating the feed on every request which is wasteful since most readers will check every 10 minutes - that could add up to be 10's of thousands of dynamically generated requests a day - and we only get a handful of posts a day. I would like to write my own that won't dynamically generate on every request, but I don't know when I'll get time. I might set this for a project to do before Christmas.