From a user's perspective, I think the calendar function would be great.  

Some suggestions as to who would want to publish their calendar.  Getting any of these automatically into a calendar would be obvious steps:

   - NYSE, London/Tokyo Stock Exchange, etc, showing market holidays.  From experience, it is a total pain having to enter by hand a list of holidays for stock exchanges in multiple countries (e.g. http://www.nyse.com/Frameset.html?displayPage=/about/1022963613686.html for NYSE holdays, not yet RSS);

   - Governments listing pblic holidays (e.g. http://www.opm.gov/fedhol/2005.asp for US federal holidays, not yet RSS);

   - Corporates listing their upcoming investor relations announcements.  Investors and the media subscribe to companies' email lists, to get announcements.  Some companies even RSS-enable their media releases.  Taking it one step further, by putting the calendar into RSS would be the ultimate (e.g. http://www.vedior.com/investor-relations/financial-agenda.asp);

   - This would be a huge project, but you could even have share registries and stock exchanges putting key dates on RSS: e.g. ex-dividend dates, dividend pay dates, share tender dates, deadlines for AGM voting, and so on for all their companies.