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This is on the Beta 4.0.4 running on iOS.
When I await Task.Yield() my understanding is that the continuation code after the await should resume on the captured context (in this case the UI thread). It does not appear this is happening, can be reproduced simply by dumping the NSThread current thread description, awaiting Task.Yield() and then dumping the NSThread currentThread description again - you will see they are different thread objects. More obviously, and UI code in the continuation will throw a UIKit exception that the UI is not being accessed from the UI thread.
Fixed in master (next beta should include the fix)