Here is the page where they do it: http://www.washingtonpost.com/wp-dyn/content/article/2005/09/16/AR2005091601983_3.html.

Actually, it only makes me wish I had written more on the piece. The key would seem to be the hot link back to the Post I'm pretty sure that's how they track you.

Think I've just found another task for my webmaster. . .