How *.drupal.org single sign-on works I believe there are many ways to achieve single sign-on within drupal. I would like to know the approach that *.drupal.org sites have been using. If I visit any drupalconf sites http://chicago2011.drupal.org/user or http://london2011.drupal.org/user it authenticates me with the user credentials given in drupal.org site. I would appreciate if anyone can point me to the modules or handbook pages on achieving this. Thanks in advance.
You might have a look at Bakery Single Sign-On System. 

It mentions "This module started at the 2009 Drupal.org redesign sprint in San Francisco as a way to manage single sign-ons across *.drupal.org infrastructure."

For more details about how it work, you can see this article that has great explanation: Single Sign-on across Sub-Domains in Drupal with No Extra Modules.