Back in March, Google announced that the CalDav API for Google Calendar will no longer be open to everyone. "CalDAV API will become available for whitelisted developers, and will be shut down for other developers on September 16, 2013. Most developers' use cases are handled well by Google Calendar API, which we recommend using instead."
Now Google changed its mind and decided to keep the CalDav API public and to make the CardDav API for Google Contacts available for everyone. "Since that announcement, we received many requests for access to CalDAV, giving us a better understanding of developers' use cases and causing us to revisit that decision."
That's a great news because you can access your calendars and contacts in future third-party apps that couldn't be whitelisted by Google because they haven't been created yet. Google still promotes the Calendar API and the Contacts API, which offer more features.
It's also a good PR move, now that Google has been accused that it no longer cares about open standards. "Google, long a champion of open standards and 'one click away' competition, has started dumping open standards, ostensibly to force fealty to Google products," says Matt Asay.