Won't happen, but they have already dropped Darwin due to the fears that the open source will be used to crack OSX to run on any hardware. (Not that it isn't already, but releasing code themselves will just make it easier).
Darwin is still alive and strong.
http://www.opensource.apple.com/darwinsource/
Apple cannot drop Darwin unless they plan on dropping BSD from their commercial OS.
Not gonna happen... so.
Releasing Darwin source code is one of many requirements they must continue to fulfill under the usage terms for the opensource code they incorporate into OS X.
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