At present the only known implementation is paired with Xcode 6 and runs on OS X only. We'll have to see if Apple open sources their front end implementation (that runs atop LLVM). If they do then it'd probably be reasonably straightforward to retarget it for other operating systems where LLVM is supported, if there's a run time to pair it with (like GnuStep).
Looks like Swift will be open source from a few tweets. I imagined as much with Apple's work on LLVM. Though the big issue is like Objective-C, it's near worthless without Cocoa and all the stuff Apple keeps closed.