I think the answer to whether the destructive "delete" should be the default action or not highly depends on the context in which the dialog occurs. If the dialog was the result of an action that expresses a clear destructive intent, like for example clicking "empty trash", the user probably knows what the consequences are, so it makes most sense to have the affirmative "yes, I really want to empty the trash" confirmation be the default action (enter confirms, space or escape cancels):
In such cases it could potentially be very annoying if there is no easy way to confirm the action that the user just performed ("yes, of course I want to empty the trash, I just clicked that, didn't I?!").
If, on the other hand, the dialog occurs to warn the user of a potentially destructive side effect of something they did, it makes more sense to err on the side of caution. A good example of this is the dialog that occurs when you close an unsaved document. In that case it is unclear if the user intended to abandon the changes, so the destructive action is not the default:
So, even though Apple's Human Interface Guidelines on dismissing dialogs states "Use a default button only if the user’s most likely action is harmless", I would say: use a default button only if the user’s most likely action is harmless and is not a clear extension of the action that triggered the dialog (e.g. it's almost impossible to empty the trash by accident, but it would be bad to accidentally lose all changes in a document after closing a window).