we did some work on this. In 2006 i did some work on webdav + ical to synchronize the calendar (bidirectional) working with the Mac OS X tools (ical program, itunes, sync with IOS) and the mozilla tools (e.g. thunderbird). For more details, see here
https://openacs.org/forums/message-view?message_id=436039)
With some newer versions of IOS (starting from maybe IOS 4) Apple changed to calDAV, which has not only much more functionality (including TODOs and group management) but as well different ways for user management, caching etc. A group of students was working on this recently, but it needs much more testing before we can release this.