If you already know C, Objective-C is pretty easy to learn.
Ruby and Python are both viable for "real" Mac apps. ADC has a few articles on the topic. MacRuby looks like it will be replacing RubyCocoa.
I would still recommend learning Objective-C though. Most of the example code you find will be in Objective-C and the books tend to be Objective-C (though the Pragmatic Programmers have a RubyCocoa book in the works. Most Cocoa apps are written in Objective-C.
And Objective-C is dynamic. Take a closer look at it, it isn't nearly as intimidating as people think. It's Cocoa that tends to have the steeper learning (or unlearning) curve.