Has there been any work in making Zimbra a SSO portal interface?

I'm constantly evaluating the options that exist on the market, and zimbra for what it does out of the box is high on my list for one of the things I'm evaluating.

A mission critical requirement where I work though is for providing SSO between different web applications. All of the web applications will be running off of the same LDAP provider, so that each user has the same username/password across all applications.

The current Zimbra demo I've been playing with on the zimbra domain has "mail" "calendar" "address book" "documents" "options" along the top. Ignoring options, its obvious that each of those are independant appliccations with built in sharing functionality for the collaboration/communication elements of zimbra.

What is involved in extending the tab-navigation metaphor along top with another tab for the (lets pull something out of my hat)"document management web-application" that would I guess create a new browser window that logs you in to that application transparently, carrying over your LDAP credentials from Zimbra.

Additionally / conversely, is there another default home page for zimbra? or is it always "mail" ?

Has anything like this been done?
Thought about?

Thanks!

I'm about to go browse the zimlets that are available, and see whats going on over there