I second and third this nomination ...
Not only would domain calendars being used on Outlook be helpful, but having a way for Admins to automatically attach them to accounts and then send account holders the link to install would be beneficial. Trying to explain to your entire office (and remote offices) how to do this is rarely successful initially.
Aside from that, I'm not sure why they bother with domain calendars if they can't be shared via Outlook, since web interfaces are not used by default - usually only as a "perk" or when they are away from the office or home. The mail server is direct linked to a client usually.