as discussed in irc:
First file shows you the desired effect: A focus empty is used on the camera properties to set a focal point
Now I save the file and reload it ... after that I delete the empty - but it's still assigned in the focal property (should have been deleted as there is no user anymore)I can't find it in the outliner neither...
On a side note, there is still a similar problem with Empties when you enable the groups duplication feature. When you stumble across it then don't wonder... I'll create a new report when I can reproduce it.