There are several good signs lately in the calendar world.
First, the Calendaring & Scheduling Consortium seems to be making progress with membership, they have some big players and are trying to recruit more.
Second, on the mailing lists related to calendar sharing via DAV, called CalDAV, things seem to be progressing fairly smoothly and they keep producing new drafts.
Third, the iCalendar simplification effort is progressing, with a lot of activity.
Fourth, I've seen participation from the "big three" (Lotus, Microsoft, Novell) of enterprise products on the lists (although they may not be members of the consortium yet).
Fifth, and most importantly - there are what appear to be many more university and open-source (the two sets often overlap) developers interested and contributing.
When a critical mass of developers on the Web decide to work on something, rapid progress is often made. In this case, I believe enough interest has been generated in "the calendaring problem" to move us much closer to the goal of true calendar interoperability. We can only hope.