[m0nastic]

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.

[sjwright]

Which implicitly means that swift code can be compiled for iOS 7. Which isn't surprising since the compiled code uses the ObjC runtime.

[chucknelson]

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.

[lcnmrn]

Swift code compiles and runs on 10.5 and up. Tested it myself.