OS X developer tools have been updated to Xcode version 3.0, Interface Builder 3.0 and Objective-C 2.0 in Leopard. I've been learning OS X development, and I found cocoacast that has three screencasts that cover these changes.
Episode 34 covers Interface Builder 3.0.
Episode 35 covers xcode 3.0.
Episode 36 covers objective-c 2.0.
