Good or bad, I'd say Apple are pretty much stuck with ObjC. Cocoa and all the other frameworks OS X uses are written like they are because of the language's syntax. This is demonstrated by how crappy the PyObjC code looks (compared to "regular" Python):
To make it work with some other language (say, C#/C++/Ruby/Brainfuck/Python or whatever) would basically be a complete redesign of the framework, which in turn would basically be a complete redesign of OS X.. And why? Objective C is a perfectly decent language.. I'm not sure anyone would be using it where it not for Carbon/Cocoa, but it's not a bad language at all