After having set up my own davical server and connecting my android phone to it, I also wanted to have the addressbook in my roundcubemail installation.
As there was no CardDAV support available, I created a small and crude, read-only carddav client plugin.

Good luck with it :-)

Update: Version 0.2 is released, which includes write support: http://blog.crash-override.net/index.php/393

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