Check out the new funambol clients for Outlook
funambol :: open source :: Funambol downloads
And test it against schedule world, or open a trial account on funambol.net, You might be able to implement a syncML server for you clients. As far as getting Calendars into outlook, I haven't tried that, I've been using Thunderbird 2.0 and Lightning 0.5 with the Scheduleworld extension and it works beautifully