The Cake Documentation Team has written an entire chapter about ACL. Here is what they cover:
1. Understanding How ACL Works 2. Defining Permissions: Cake's INI-based ACL 3. Defining Permissions: Cake's Database ACL 3.1. Getting Started 3.2. Creating Access Request Objects (AROs) and Access Control Objects (ACOs) 3.3. Assigning Permissions 4. Checking Permissions: The ACL Component
They have done a good job explaining the not so easy stuff :)
Update (2006-08-11): Link fixed.