I don't buy that edge sockets and DVD are a major factor.
Perhaps shouldn't have limited to the edge and DVD. The HDD , RAM , and CPU play a role but those can be negated.
People have external backup drives connected almost 24/7 or at least daily. Having an internal TM volume would be much more valuable than an internal ODD,
An internal TM volume in a mobile device is a dubious idea. The major reason why folks shift to SSD in mobile products is to avoid the disconnect between walking around and spinning hard drives. A HDD that is being bounced around as being your primary source of back-up is a bit off. It is nice just as long as don't think about just how vulernable your back-up drive is located there.
Frankly, I don't think most folks back up much even with stationary drives. Time Machine makes it easy by many don't. There is an even smaller number of folks backing up to either two time machine stores or back-stopping with incremental clones in addition to time machine. I seriously doubt that is mainstream. Nor an idea Apple would encourage to go mainstream.
No, the main reason why the 13" MBP might outsell the Air is disk space (particularly disk space per dollar)
I suppose that will last this year. Apple bought a flash controller IP company. I suspect they are soon going to deploy flash with "good enough" capacity and just leave those with high bulk and strong $/GB demands to small external drive powered off the Thunderbolt port. Just like how the Ethernet can be pushed into a dongle, 2.5" HDD can just as easily be pushed into a dongle once Apple turns that into "the few who need it".
and to a lesser extent more RAM and faster CPU (and until now also faster
RAM is much of an issue if comes with 8 or 16GB by default. If Apple cut the "extra" Apple tax slightly for the 16GB variant all the more true.
I/O with the Air defacto limited to 100 MbitE and USB 2 because of lack of affordable TB stuff).
Errr? The Air now has USB 3.0. and the 1GbE will be (or is ) $29.99. For the latter, just how much affordable you want it to be? [ the 1GbE dongle isn't a BTO option for the MBA now but I suspect that is a hiccup that Mountain Lion will cure. It is just a matter of drivers and configuration testing. The dongle's driver update was screwed up some folks on Lion. ]
I don't think Apple is going to measure against what the MBA 13" and MBP 13" sales were before. I'm talking now that the prices and some of the feature sets have been evened out it is going to be much more clear which one has more inate appeal over the medium term.
The MBP 13" an MBA 13' were tiered slightly above each other before. Apple could go back to that with the MBP 13" staying at the lower spot and the new MBP 13" taking the higher price tier structure. The MBA 13" could just peter out after about a year's run until the next update cycle if it is the weakest out of the three.