When we (Acquia) started planning our hosted Drupal service (Drupal Gardens) a year ago we had to take the call of developing in Drupal 7 or Drupal 6. I don't think this was ever really in doubt, but the decision to try to build a product on Drupal 7 core at that stage was certainly a risky one. Why? there were almost no contributed modules or themes and the architecture and APIs were changing daily. Acquia's business is squarely Drupal. Drupal support, Drupal promotion, Drupal hosting, Drupal polish, Drupal architecture, Drupal Drupal Drupal (you know the song). So even though this was a risk, we knew that the benefits to Drupal and Acquia would be too great to back down from it.