If you know Ruby and you want to write applications for Mac OS X or the iPhone, you're in a great place to start! This concise and content-packed Objective-C course will teach you what you need to know to start programming in Objective-C.
Objective-C and Ruby both share many features, thanks to their shared inspirations from Smalltalk. You can use your knowledge of Ruby (or even Python) to understand many of the core concepts of Objective-C. This screencast also covers crucial concepts that are not part of Ruby, such as memory management and the extensive use of the delegate pattern. Better yet, you'll learn how to do Test-Driven Development in Objective-C as you learn the language.