It's amazing that Apple managed to go from hatching the idea in mid-2010 to releasing a fully working framework 4 years later, with tight IDE integration, huge amount of testing and compatibility without a single leak (that I've heard of).
The existence of a programming language named Swift under internal development at Apple had leaked. What it was for wasn't clear – it certainly wasn't clear that it would replace Objective-C.
It does (direct interop with ObjC was obviously a design constraint; just look at function signatures), but there was still a tremendous amount of work to make a language one guy started on in 2010 into a beta today.
If they're willing to allow developers to release an app written in Swift when the beta ends, I'd say they're pretty comfortable with it in production. If Swift is really four years old already, who knows what components could be already in Swift. And it's not like Apple jumps ship overnight - iTunes spent nearly a decade as a Carbon app after introducing Cocoa.
Not that it's particularly complicated, but they mentioned yesterday that the updated version of the WWDC app (which they released right after the keynote) is written in Swift.
I don't know if that means "entirely written" or some parts, but they at least called attention to it.
When they talked about the WWDC app being written in Swift they also explicitly mentioned that Swift will run on iOS 7 and 8 as well as OS X Mavericks and Yosemite.
