I like this idea. For the purposes of a forum, you might want it to be more iconic (read: smaller), so maybe a really rough bar graph of like one bar per week over the past month or so. At that rough of a level, you can also easily push the generation to a weekly cron job (rather than trying to keep up with every new post).

Thanks. I've been thinking about your advice and I made a quick mockup of sparklines which show 4 bars, each representing a week, where height is average popularity over that week, and darkness of the bar is how many posts there were that week: http://www.tipjoy.com/our2cents/2007/09/sparkline_update.htm...