The actual guidelines state 12" or smaller can stay in your luggage, however, the individual screener is given wide latitude as to whether to request it be removed and scanned.
When I travelled more frequently and owned the 11" Air, I was only stopped once and asked to removed and scan it once my bag was screened. I think most operators will let it go.
From the TSA Blog.
For laptops that need to come out of your bag, we describe them as a standard size laptop which loosely translates into approximately 12x14 or larger. Were not measuring every laptop that comes through the checkpoint but that is the general dimensions of what we consider to be standard size. Also, the larger the laptop, the more stuff that can be hidden in it. So if your laptop is approximately 12x14 or larger it must be removed from the bag, but remember, if you choose to leave the smaller ones in your bag, our officers still need to be able to see clearly on the X-ray what else is in the bag with your laptop, so there is always a chance they might ask you to remove it to give them that clearer view. ~
Since the new MB is approx 11"x8" it should not be a problem leaving it in your luggage.
Cheers,