I’ve just installed the most recent version of Calendaring and ATContentTypes (both in the collective), and have to say it is very slick what you can do with this product. I can do the following using iCal and Plone:

• Create a calendar in iCal and publish it to a Plone site via WebDAV. The events appear in the calendar view and as normal Event objects
• I can add a new event in iCal and when I refresh the calendar, it sends the new event to the Plone site. It’s smart enough to know that if I edit an existing event to update the event instead of duplicating it. This must be because each event has a unique ID.
• Using iCal, I can subscribe to a calendar that is being hosted on a Plone site. This can be done by clicking on the Event icon when you are viewing the calendar, which launches iCal and auto-subscribes. This works better in Safari than in Firefox. Firefox wants you to download the .ics file first, after which you would double-click on it to open
  it up in iCal. A workaround for this would be to change the http:// to webcal:// which would tell Firefox to use another program (i.e. iCal) to handle the file. (DONE)
• I can add an event in the Plone interface and in iCal do a refresh of the subscribed calendar. The event that I just added in Plone will now appear on my iCal calendar. I can also set it to auto-refresh every 15 min. This means that the artist’s fans don’t have to keep clicking on refresh, nor do they even have to visit the artist’s website. All they have to do is launch their iCal program and it will automatically grabs all the most recent event postings from the artist’s calendar hosted on the Plone site.

What is missing:

• There is no two-way synchonization. That is, if I publish a calendar from iCal, and then add an event to that calendar in Plone, it won’t be synchronized back to the iCal calendar that I have published. You can certainly add events both in the Plone site and via iCal, and those who have subscribed to the calendar will see these events (no matter where they were entered), but the person who published the calendar will not see the events. A simple workaround is simply to subscribe to the same calendar that you have published, and use that one to view ALL the events (incl. the ones that are posted from the Plone site).
• Recurring events don’t work in Plone. If you make a recurring event in iCal, it will show up as a single event in Plone. UPDATE: See this post in which Tom Hoffman mentions that Programmers of Vilnius will be adding recurring events functionality as part of a contract job.
• Filtering the events by the event type / keywords. The CalendarX product does this, so I don’t think it would be too difficult to add to Calendaring.
• Nesting the calendars. For example, an artist is in several bands and if you subscribe to a single band calendar then you only see the events of that particular band, but you can also subscribe to the artists’s calendar which will show you the events from all the bands that the artist is in. UPDATE: See Jeff Kowalczyk’s post regarding this.