I think the 12" lend themselves to the 4:3 ratio...
Netbooks these days are 11.6"..at least the Asus ones I know.
Ah yes, that makes sense.
I hate that ultra portables have become synonymous with the poorly made tripe that gets squeezed out by the netbook market. Apple will have to be careful to craft it with better performance than netbooks and higher quality components. I dont mean an alu shell, and precision engineering, thats a given. I mean shoe horn a discrete graphics option, a really top notch display and SSDs only. That would definitely help legitimise the price premium that it will obviously have.
The current air does have an identity crisis in that the footprint is so similar to the macbook, that very few of the laymen in store will appreciate the volume difference between the air and the macbook, when they're sitting on opposite ends of a table.