From the Official Google Blog:
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. If you’re a developer and the Calendar API won’t work for you, please fill out this form to tell us about your use case and request access to whitelisted-only CalDAV API.
Remember when Google announced they were dropping Exchange Activesync support for the CalDAV standard, and the Internet cheered for the use of standards?