In the current issue of SitePoint Tech Times (#90) Kevin Yank tells us about how websites can abuse our privacy by using css and some server side scripting to know when you have visited a particular, different, site.

Its a clever, but technically simple, matter of using css to provide a script as a background-image on visited links, and then create a link to a competitor’s (or other) site, to see if visitors have been there - the visitor doesnt even need to be able to see the link and will have no idea this is happening.

Kevin goes on to say that "The best way to minimize the impact of this particular issue is to set to its minimum the number of days that your browser remembers visited sites.".
I disagree. If this is a serious matter to you (and it isnt to me) then a better method would be to have a user style sheet that has a style defined along the lines of:
a:visited { background-image: none ! important; }
would prevent this particular implementation.
This will have side effects, as you will miss out on some of the cleverer effects that are being developed recently, but if your privacy is that important to you, then you may decide it is worth it.

Generally, privacy is being a more and more important issue for more and more people. "Tricks" like these will become more and more common.
This sort of trick may well be the sort of thing that a thoughtful site developer will disavow in a site privacy policy.