I was just the messenger here and not involved in the project, so other people might give you a better answer. I assume that development takes place constantly (there is a steady flow of Git commits), but I do not know how usable it already is.
What I heard some time ago is that there is still is no definite plan how to introduce it so that it plays well together on the various platforms, i.e. that Swift programs using the new Foundation are really cross platform (except obvious differences of the platforms) without any hiccups like platform specific code (e.g. imports) in the application. Update: See this topic.
But as I already mentioned, others might know more.