Heather Solomon has created a great series on her blog titled "Branding SharePoint". This is definitely a required reading for developers and GAs serious about giving their corporate SharePoint instances an identity.
Branding SharePoint - Part 1: Designing your SharePoint Site
Branding SharePoint - Part 2: Creating the Design in SharePoint
Branding SharePoint - Part 3: Changing the look and feel of SharePoint components - Intro and Web Parts
Watch for more articles in this series via her RSS feed