For several versions now, CiviCRM ships with version checking mechanism that pings our server from time to time (every time you visit the Administer CiviCRM page of your install, but not more often than once a day) and lets you know if there’s a new major stable version available for download. (This mechanism can be turned off by visiting Administer CiviCRM → Global Settings → Miscellaneous Settings.)
From CiviCRM 2.0 on, we’ll be adding three parameters to the version pinging mechanism – these parameters are the version that you have currently installed (so that we can guesstimate the adoption rate of new CiviCRM versions), the CMS you’re using (so we can see the Drupal- vs. Joomla-based usage numbers) and an MD5 hash of your base URL, so we can guesstimate the number of separate CiviCRM installs in the world.
Note that MD5 hashes are not reversible (in particular, an infinite number of strings hashes to any single hash value), so we can’t reverse-engineer where your site is installed; this change doesn’t lessen your anonymity in any way. (If you’re a PHP coder, feel free to review the relevant commits.)