Federighi said on Gruber's podcast that the Dock team completely rewrote Dock in Swift. However most of the other OS X teams are only testing the waters. A big part of it is lack of 32-bit support for Swift apps.
" the team that does the Dock and the window management on OS X, implemented all their new features for El Capitan in Swift and started mass-converting all of their code, and say that they couldn’t imagine going back and that they’re more productive with it."