Scott Stevenson has written up a great new tutorial: Introduction to Cocoa Graphics (Quartz) for his very helpful Cocoa tutorials web site: Cocoa Dev Central.
There is some great stuff here. I truely wish that I had this resource back when I started Bullfrog.
Scott skillfully walks the reader through a simple illustrated overview of the basics of Quartz drawing with Cocoa and Objective-C. He promises there are more tutorials to come, delving further in to the world of Quartz and Cocoa Graphics.