Interesting thought. It would be a great way to surmise where your advertising dollars or link-building time would best be spent. And a nice way to see what competitors were being shopped before and after you. I've always wondered where my visitors go after they visit my site...other than to the refrigerator or to pick up the crying baby.
I think Amazon does a little of this in their alexa rankings.. for example, if you search for traffic rank details.. there's a "related links" section that tells you a handful of other sites that visitors have been to.