Seems logical...
In one or two more generations, when SSDs are cheep enough so that 256GB can be standard in the 13 inch $1200 base model, I see apple dropping the "Air" name and simply having 11/13 inch MacBooks, 15/17s would retain "Pro" status. Right now there is a lot of user base crossover between the 13 MBA and 13 MBP, most people that currently have 13 inch MBP would probably be better suited with a 13 MBA. If you consider the optical drive dead (and apple does) , then the only thing keeping the 13 inch Air from completely replacing the 13 Pro is cost and HDD storage space. Once the storage/price problems are solved by cheaper larger SSDs, there would be no reason to have a 13 inch "Fat" macbook and 13 inch "Air." Lets be honest, for most people there is very little advantage of having the slight CPU speed bump that having more cooling in the larger 13 inch would allow, the real limiting factor is of these notebooks is the GPU, and that currently is the same in the 13 Pro/Air anyway.
Late 2012 or Early 2013 line:
11 inch MacBook: Base 4GB Ram, Dual Core 2.0 Ghz Ivy Bridge, 128GB SDD (optional 256GB), Integrated graphics
13 inch MacBook: Base 4GB Ram (optional 8GB), Dual Core 2.2 Ghz Ivy Bridge , 256GB SDD (optional 512GB), Integrated Graphics.
15 inch MacBook Pro: Base 4GB Ram (optional 8GB), Quad Core 2.0 Ghz Ivy Bridge, 256GB SDD (optional 512GB/1TB), Dedicated Graphics.
17 inch MacBook Pro: Base 4GB Ram (optional 8GB), Quad Core 2.2 Ghz Ivy Bridge, 256GB SDD (optional 512GB/1TB), Dedicated Graphics.