I am currently operating my own CalDAV Server. I am running http://trac.calendarserver.org/. However I am thinking about using Ubuntu One but for that I want an open protocol which allows my mobile phone to sync with it and is integrated into many applications on many platforms.
I could not find if/when this feature is planned for Ubuntu One?
Btw. the calendarserver is the implementation powering Apple's me.com or in the future icloud.com. The other option is http://www.davical.org/.