That's an interesting problem, and I think it would be a proper application for these "cookies" as I'm skeptical about their long-term use. I guess you'd just have to toggle a varible between "seen" and "notseen" as they left the site.

I have an example, here, that shows the "cookies" working. It "remebers" the hexagons that you selected. You can click a pattern and return a month later and its still there. I though of making it into a flash lite-brite but that won't pay the bills.

http://home.twcny.rr.com/jpitcher/hex.swf