Just a heads up on this, I've just been playing around translating some of my iOS View Controller code for my app to Swift. Its been a bit of a faff and a little learning curve but I so far much prefer Swift to Objective C. Only reason I looked into it was coding in Objective C was starting to annoy me. Although you can't interop between Swift and C++ you can between Swift and Objective-C++ so it does not make it much different to Objective-C and Swift interop. To call Objective-C classes from Swift you need a <ProjectName>-Bridging-Header.h (which Xcode will generate for you and you just fill in the gaps). To call Swift from your Objective-C or JUCE Objective-C++ (.mm file) you #import <ProjectName>-Swift.h which Xcode will magically create for you (i.e. you do not need an actual file with that name).
Once all thats set up you can happily call Swift classes as if they were Objective C, and vice versa. Some hoops to jump through and things to learn especially with regard to init functions and UIViewControllers but feels worth it for a much nicer coding experience when there is a need to do things in iOS native land.
[EDIT: I realise this is not applicable to the "Open Source" aspect of Swift - only iOS and OSX development]