Public bookmarks, Private tags

Has anyone come across a bookmarking site that allows us to make bookmarks public, but attach private tags to them?

Here's the use-case:
Tags in my opinion are small bits of information that we attach to the entity under consideration. Now the entity itself may be public, but not the information that I attach to it.

To be more specific, let me tell you where I felt the need for this. When chatting with , I came across several books in Amazon. I started bookmarking these in delicious and attaching the tag bibliophile to them.

Then came a thought. How about storing information like whether I have read this book or not, what the number of this book is (I number all my books) and other information which might not be worthwhile to make public or for some reason does not seem appropriate.

In terms of implementation, I guess it is quite simple. You need to now track each tag and see if it is private or not (an extra field in the database) and then display them accordingly in the UI.