You know all joking set aside, this is a very important subject; Microsoft really needs to hone in on the legal definition of its product.
According to the TSA's carry on page,
"Laptop computers, full-size video game consoles, CPAP machines, full-size DVD players, and video cameras that use video cassettes must be removed from their carrying cases and submitted separately for x-ray screening. Small and portable electronic items (smart phones, tablets, portable games, etc.) do not need to be removed from their carrying cases or carry on bags. "
It would seem then to leave (as usual) a great deal of discretion to the TSA official who happens to be near you. For a frequent flier, I could see the not knowing of this issue being a real pain.
Even flight attendants can cause you some trouble.
" When we landed I was told I had to shut the Surface Pro 3 down. I protested in a polite fashion and then removed the tablet from the keyboard to prove it was a tablet and not an “evil” laptop. The flight attendant seemed quite surprised by my actions – she said she had never seen a tablet that large and told me to use it with keyboard off until she went further towards the back of the airplane."-
Surprise! Microsoft Surface Pro 3 Not Airplane Friendly