Thinking about it overnight -- I think that I would express this "private tags" notion as "bookmarks" or "favorites". That more clearly says that these are *my* links to *your* stuff, and are private to me.

Looking at it that way, it's actually a more compelling story: I suspect we'll eventually want to implement it, and I've put it onto the Features list. Thanks for the suggestion -- that one hadn't occurred to me before. But it'll probably be a ways off...