If you're going to write Mac software, just learn Objective-C and Cocoa. Otherwise, you will not create applications that "feel" the right to Mac, and customers will not be delighted.
If you are going to write software for the iPhone, you will need Objective-C for UIKit, unless you do everything in OpenGL.
I would also recommend learning C ++, as it is standard, and it has a lot of libraries written. But I would personally avoid C ++ for the direct user interface on a Mac, Windows, or iPhone, but would recommend it for running in the background.
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