Apple had CalDav based calendars for MobileMe and iCal in public beta in July and out of beta in October. The CalDav calendars are wonderful because my wife and I can have a shared calendar.

As of December: Known synchronization issues between BlackBerry Desktop Software and the new MobileMe calendar from Apple

“This is a previously reported issue that is being investigated by our development team. No resolution time frame is currently available.”

The work-around is to create a local calendar that the BlackBerry Desktop Software can sync with. If an event is created on the BlackBerry, after syncing to iCal the calendar for the event can be changed to shift the event from the local calendar to a CalDav calendar.