Apple has just announced right now that the Swift programming language will become Open Source late 2015. So we can expect that the Swift programming language will be running on Windows, Linux and OS X, iOS, etc.
Anyway, the OS API’s will not be available. So you can not write an OS X Application in Swift and run in strait on Windows - but the Swift programming language will become more and more powerful on other platforms nut just Apple's OS X and iOS.
println("Hello from Swift. I will be Open Source soon late 2015!")
Swift home — https://www.apple.com/swift/