This exact question is why I think the rumor about a 12" (11.88") Air is possible. If they make a single 12" Air with retina display, and drop the 11 & 13 model Airs, then this makes sense.
12" Air - retina
13" Pro - retina
15" Pro - retina
The biggest difference between the Air and the Pro will be size and power (as stated in an earlier post) to keep it lighter and with better battery time. I do wonder about price, though.
And as they always do, I believe, they will keep the 11" $999 model - non-retina - for at least a year so they can offer the "cheap" solution. Like they did with the non-retina MacBook Pro.
Admittedly, I will miss the 11" model as that is, for me, the sweet spot when it comes to perfectly sized laptops to get day-to-day average business stuff (and writing) done. Though, I am a computer nerd and an Apple fan, so I will want the new Air I am sure. Even if it will be bigger.