As Glen writes in his Exchange Dev Blog:
I’ve gone a bit RSS mad of late. For today’s edition I created an Event Sink for public folders so when a new mail or post arrives into that folder it will fire a script that updates a feed with the last 7 days of posts. I guess the cool thing here is you can stick this on all the important public folders in your org and then use one Web page to aggregate across all the feeds and present this back to the user as one single Web page. Also, it lets people subscribe to folders and receive notification of updates, etc.