Looking back at the keynote for the Air when Jobs introduced it years back, its primary focus was to be the world's thinnest notebook. At that point, you had:
MacBook - Consumer
MacBook Air - World's Thinnest Notebook
MacBook Pro - Professional
At this point, the MacBook is now serving the purpose of the thinnest notebook that they offer. I am guessing it will be.
MacBook 12" - Thin/Light Use
?
MacBook Pro 13", 15" - Professional
It is clear to me that there does need to be a third category to fit consumer products, i.e. lowest cost. It was a really big deal and something that Jobs talked a lot about in the keynote to bring the pro level features of the MBP ($1,999) at the time to a consumer level computer in the MB ($1,299) and that was emphasized heavily. When the original Air was released, it was $1,799!!! so it is actually quite similar to the current rMB in terms of its DNA. It is seeming like it was named wrong, though, and really should be the new Air.
It really does seem to me that the MacBook should in fact be called the MacBook Air. with a third computer called MacBook introduced for consumer/education, etc. So it will be interesting to see what they do. Man I am looking forward to this event.