[drupal-support] How to handle the www subdomain?

I have an apache server running Drupal 4.6 and php 4.10. I notice that when my domain is accessed via www.mydomain.com, it has a different user session than my domain at mydomain.com (with no www subdomain). In other words, I can be logged in at mydomain.com yet logged out at www.mydomain.com and vice versa.

I notice this isn't the case at drupal.org, however. When I log out at www.drupal.org, I also get logged out from drupal.org. Does anyone know how to accomplish this? Do I need two settings.php files and change a php setting or two? Or is this an apache configuration issue?