There are a number of interested people at the W3C. The Semantic Web has an RDF Calendaring and Scheduling Working Group.
The charter is available online. Their work includes a standard data model, a protocol for calendaring and group scheduling operations (the "Calendar Interoperability Protocol"), and a protocol for managing calendars over the Internet (the "Calendar Access Protocol").
They list plenty of prior work in this area. In particular, they make mention of these earlier efforts:
The ILRT is the home of a number of people involved (or interested) in calendaring; in particular, with the oh-so-attractive notion of having something else organise the hum-drum details of their lives :-) Most work here is aimed at the same target as the W3 Calendaring group, for obvious reasons...
I'd like to make this list reasonably complete. If there's something missing from here, drop me a line and I'll add the appropriate links.