Apple have announced iCal in Mac OS X Leopard will support CalDAV servers. They’ve also announced they’ll be shipping an iCal server with Leopard Server, probably based on the Darwin calendar server they’ve just open sourced.
They’ve also joined the CalConnect Consortium (which includes groups such as Boeing), who conduct interoperability testing between calendar implementations. Amusingly many large organisations would be happy even if they could work out when people are free or busy, let alone anything more complex.
So once you’ve got the servers up and working you can try connecting to them with Chandler, an open source personal information manager implementation.