Nice mod, too bad the red line is on the wrong side of the "paper".
Actually, there are two red lines. The darker red line is at the top of the keyboard towards the "spine" of the notebook. The faded red line at the edge of the page is the assumed "right margin" or the left margin of the next page that "bled through" (see below).
I've had enough composition notebooks in my life (plus the one I was designing from) to give me really awkward flashbacks as I was designing it.
That's pretty good. The problem with all these gimmicky products for the Air is that they are funny for like 5 minutes, then you realize they just look crappy and take away from how beautiful the air is. Like the envelope sleeves.....