There's such a thing as iCal for Calendar sharing. It's a standard protocol and clients exist that handle it. Vista comes with one. Sunbird is another.
Yes, Thunderbird *does* do what most people need. Most people need access to E-mail. Not to Notes or Calendars or weird IM systems or whatever the hell else Outlook does. But, OK, integrate mail with calendar: Google Mail + Google Calendar might be a decent jumping off point. Obviously, Google's stuff is at the moment google-only, but certainly, they know how to build and scale their applications and they do have a history of packaging their solutions for smaller operations (Google Search Appliances). Authentication would be an issue, but LDAP is an open system. It shouldn't be that hard to figure that one out.
Really, messaging, address books and calendars are all things that work well on with a thin client and a thick server. A web browser is a great universal client for everything else anyway. There's great, light authentication systems in your browser. Just displaying X messages or dates at a time instead of sending them all to the client is a win for all parties. It certainly SHOULD be possible to build something decent from all the wonderful AJAX building blocks that are floating around nowadays.