Looking at this demo of Tapulous FriendBook, I can well imagine, but have no knowledge of, a similar app coming for calendar sharing directly between iPhones.
I think of the possibilities here like reviving the old Palm-style calendar-event beaming ritual that River and I found so useful, with the added benefit of automatically syncing the ritual with calendars in the cloud. [Memo to Palm: This is what you should be doing.]
The Tapulous perspective is the right one. Sync the personal devices with each other. Web sites tend to be headed the other way: Give subscribers to the event an ever-increasing list of calendar platforms (Outlook, Google, iCal, etc. etc.) to export the event to (or subscribe in). This perpetuates the calendar swamp (or silos, if you will) and creates a confusing user experience because who wants to scan that growing list of calendar platforms every time you want to add an event?
The Tapulous scenario represents one type of sharing -- between two individuals. The Web 2.0 calendar scenario is another type of sharing -- getting calendar data out of the Web 2.0 site and into some other calendar. We somehow need to integrate both scenarios in as simple a manner as possible, without locking ourselves into any one calendar platform. If the Web 2.0 sites don't agree upon a standard, then something like Tapulous will probably become a default platform in the long run. But then we're locked into the iPhone. Wouldn't Apple love that.