ScalableCalendaringAndScheduling

Centralized calendaring and scheduling are somewhat mature. Exchange server basically works. So does yahoo calendar.

But what if I want to schedule a meeting between three people in my company, four in a company that uses lotus notes, three that use yahoo calendar, two that use PDAs, and one that uses emacs diary?

I need a scalable calendaring and scheduling solution; scalable in the sense of AnarchicScalability of the Web.

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more on reconcilliation rituals (PaperTrail style).