Ok, I've done a little digging, and it appears that the only unported packages that the calendar package depends on are acs_events and acs_object_util. I can't find any references in the code to the other packages that Jon mentioned.
For some reason, acs_events didn't make it into our cvs archive, and it doesn't appear on the status list. I did a checkout of acs_event from cvs.arsdigita.com, and I'll add it to our cvs archive when I get a chance.
The acs_object_util package is part of the site-wide search package which is yet unclaimed. If we're going to port the calendar module, we need people to also port site-wide search and acs_events modules.
Even if the calendar module doesn't scale well, I still think that we should port it. Comments in the code indicate that the scaling problem is related to permissions checking, which is something that can probably be fixed with some redesign of the permission system. This is something that Ben is already investigating.
