I'll admit I've always had a weakness for the depth argument; interestingly, this has parallels to cyborg-issues. A grandfather clock does not make you a cyborg; a brainport does. How about a smartphone? Is it close enough to the body yet? How about a pacemaker, an enhancement inside your body? How about a cassette walkman? Where do we draw the line? Because it doesn't always seem to be at the skin.