I completely realize that there's no rigorous scientific method to this, but I'd thrown it together in an effort to convince someone that Drupal was not a risk and was on the upswing. My argument was that the real strength of a CMS was evidenced by its developer community.
WordCamp SF (May 2010): 770 attendees
DrupalCon SF (Apr 2010): 3000 attendees
So... assuming Drupal popularity is growing exponentially, by DrupalCon Chicago in March 2010, we'll have at least 4000 attending. Note that North American DrupalCon attendance is typically above the trendline :)