Evert Pot has posted to his blog about a CalDAV library he's worked up to allow for read/write access to calendaring that's compatible with the normal CalDAV systems.

My main goal is to create a general-purpose CalDAV library, rather than a consumer-ready server app. I'm hoping people will be able to use this library to enable CalDAV functionality in their existing applications.

You can get more information on the installation of the library from the SabreDAV wiki and to get at the code, you'll need to do a subversion checkout to get it.