There is no discussion of the privacy issues, which seems like a huge omission. I wouldn't use this on a website unless you like playing with fire.
http://www.mikeonads.com/2008/07/13/using-your-browser-url-h...
Summary: To mitigate, either (a) tell your browser not to save any history, or (b) in Firefox, go to about:config and turn off layout.css.visited_links_enabled. (Chrome doesn't seem to have about:config ...)
This should probably be the default ...
Another workable approach are keeping a full history graph, and only showing visited links on the site where you originally clicked on them.
Or, the browser could highlight visited links in a way that can't be detected with JavaScript code.
This isn't hard to fix, so obviously we just haven't been complaining loud enough.