I am not familiar with the complete functionality of the Zimbra calendar but I had some thoughts of what may be useful if not yet available:

-- Are there methods for importing calendars? If I were a student again I would like to be able to have the registrar send me my schedule as a calendar. I can then just import it to the application. Perhaps there is another way for sharing calendars?

In addition to lectures we may want to include office hours, course outline for each lecture, a suggested progress plan/time-line created by the course developer/instructor and of course assignments. This may appear to overlap too much with a course management web application like WebCT and Blackboard (for example) but scheduling is the shared component.

-- Can a calendar be shared with all or certain people/groups?

-- Can I subscribe to a calender like I do with RSS feeds and podcasts in itunes? This will allow me to subscribe to a department, university, course or seminar calender and automatically have it updated. The option to select which calendar viewed is already available.

I noticed that google calender allows for a single click to add an advertised event to my google calender. This is also a nice feature.

Maybe this would develop into a calendar browser?