This talk introduces the Objective-C language, compares it to Java, and discusses why most Cocoa developers prefer Objective-C.
A PDF of the slides for the presentation are available. The talk highlights unique features of Objective-C from the perspective of a Java developer, including what is cool (like dynamism and flexibility) and not so cool (mostly the stuff inherited from C). Includes some discussion about the process of selecting the right technologies for projects.
Overall, it's a good high-level overview, although nearly all of this material is covered in the Objective-C Overview screencast.