User rights: allow to share with one that have a special link
I suggest to introduce a new rigth system as google (or picassa, doodle...) do: authorize people who have a link to access to this information.
With a special link (generated id as google doc), it should be difficult (but not impossible) to someone to have this access.
If it is done, this should be mentionned (in tooltip for example).
For example, this could useful to display kune calendar into google calendar.