The cas module developed by the Drupal community and information about it can be found at http://drupal.org/project/cas

Improved instruction can be found on the GT Drupal website (http://drupal.gatech.edu/wiki/configuring-cas-module)

OIT is currently using a slightly modified version of this module which is included here. The tar file posted here includes the phpCAS library. You will need to place this in your 

<drupal root>/sites/all/modules directory. 

Just like other modules you will need to enable in the modules page in the administration area (http://mysitename.com/admin/build/modules). Once enabled, you will now have a new sub menu under User Management called "CAS Settings."

CAS Server Settings:

Version 2

Server: login.gatech.edu

CAS port: 443

CAS URI: /cas

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User Account Settings:

Important to check "If Drupal is not the user repository, should cas highjack users with the same name?"  and "Should Drupal user accounts be automatically created." If you don't check this then you will have to create a user in Drupal before a user can log in via CAS. Of course in some instances, you might want this control.

It is important to note what the "Auto-assign users to the role(s):" is set to. This is a nice way to pre-grant access to users. However, keep in mind that the entire GT community has a CAS login ID. So you would not want to have the just anyone with higher than the "authenticated user" privilege. And you do want to make sure that role has limited access. 

If you need to grant additional privileges you NEED to create a new role. The "authenticated user" will in most cases only be able to leave comments if that is enabled on your site or to via content you only want viewable by the GT community.