I have seen a phpbb hack which is similar to this one, but it is a little bit more user friendly. The hack is called topic calendar:
http://www.phpbb.com/phpBB/viewtopic...ender+calendar
The difference is that you can edit any existing thread to make it a calendar event. You just edit the thread and select the date and time and the event is registered. And it is very easy to delete the event. Just delete the date/time and the threadc is not an event any more. Then it is just an ordinary thread again. The same applies for new threads. If you select a date it will be listed in the calendar, otherwise it is a forum thread only.
The problem I see is that this might have a big performance hit on large forums, since each thread needs to be checked to see if a date has been set.
What do you guys think?