The TSA will clearly have issues managing this. All MacBooks look the same to them. They can't currently manage basic stuff like "baby formula might need different rules than our 3oz standard for liquids." Expect pressure on Apple to make it a non-issue.
They even said that laptops with replaced batteries are fine...how on earth are they going to enforce that? What's stopping me from saying "Oh yeah, I got the battery serviced last month"?
Given the reactions to the Boeing news that I've heard, it would be surprising to me if people would risk jeopardising the safety of a flight to bring a personal item. However, people do remain people and act accordingly.