I don't know how long this has been available at del.icio.us, but I just discovered that you can opt to *not* share your bookmarks on a case-by-case basis — for example, perhaps you don't see a need to share links to job descriptions you've bookmarked. Not that I'd know anything about that.
In case you, too, have missed this — in your account, go to the settings page, then under "experimental" select "private saving" and check the box. Next time you go to post a link, there will be a checkbox next to the url field that says "do not share." Voila! When you log in and view your page of links, anything that you have marked private says "not shared" in the place where del.icio.us would normally tell you how many other folks have saved the same bookmark. When someone else views your bookmarks, anything marked private doesn't show up at all. See?