I've just installed and configured a Mac Mini Server for a customer who wanted to stop using Google Apps for their email.
The plan was to also use the new server's calendaring service but we've hit a small snag: the staff all use iCal on Macs and Nokia E71s and there doesn't seem to be a way to use a single calendar for each person that syncs with both.
iSync/iCal doesn't appear to allow their manually-synced phones to sync with the CalDAV calendars on the server, only with local calendars on their Macs. Nor does Nokia's organiser software support CalDAV but rather SyncML for live networked syncing.
I was wondering if there's a plugin for iCal Server that would provide a bridge to a SyncML service I could run on the server... or anything else that would work (preferably FOSS)!
There appear to be many production-ready ways to do this with Google Calendar.
However, once Google Calendar is out of the mix, your only hope is this: http://developer.symbian.org/wiki/index.php/CalDav
Unfortunately, this project isn't just not-production-ready: it's also sponsored by Sun Microsystems. Now that Sun is slowly dissolving into the stomach lining of the Oracle Corporation, nearly all of Sun's prior open-source project commitments have been called into doubt. I would imagine that this is no exception.
Your best (only?) bet is to continue using Google Calendar as part of the calendar sync process, even if you have moved e-mail itself to an in-house server.