Most of you are aware that CiviCRM collects version, CMS and an MD5 hash of the base url from a CiviCRM install. We discussed this feature in the blog post: Extending the Version Check Mechanism in CiviCRM 2.0. The CiviCRM admin can decide not to participate in the ping back mechanism.
Here are some useful stats that we've collected using this feature: We've got close to 4000 installs running CiviCRM v2.0. Approx 66% are Drupal, 34% are Joomla. We have 133 installs testing various versions of 2.1 alpha, 98 Drupal / 35 Joomla.