From version 3.0.6 phpBB finally supports ATOM Feeds! To enable this feature go to (GENERAL / Feed Settings). The tricky part is making the feed icon visible next to each of the forum icons. There's a piece of code that handles this in the default Prosilver style, however, it's been commented out for some reason. Bellow I'll describe how to add the icons to an older style, which was designed without feeds in mind (seems to be common case these days):
1. Copy the feed icon from Prosilver to YourStyle:
from: /styles/prosilver/theme/images/feed.gif
to: /styles/yourstyle/theme/images/feed.gif
2. Open the style template: You can use Administration Control Panel:
STYLES / Themes / Edit / forumlist_body.html
3. Add the code responsible for feed icons: Put it between lines (Line 1) and (Line 2)