This week's launch of Google Calendar has raised some interesting discussion about how 'nice it will be' when they offer an API for it. Charlene Li at Forrester discussed this and Dare Obasanjo followed-up with good points as well.
In looking here, there are currently five APIs tagged with 'events', the closest tag to 'calendaring' here at the moment. Most of these are indeed events-focused, but the API from social calendar service 30 Boxes is there as well:
Given this flurry of activity in the calendaring space, I suspect this list will grow before long.