Currently some differentiators between the 13" air and 13" mbp:
* size
* weight
* screen resolution
* nvidia graphics in the air supposedly perform better than the intel graphics in the mbp
* air uses core2duo, pro uses corei5/i7 (would change with hardware refresh)
* i have heard (unsubstantiated) that the mbp can get fairly hot, while the air seems to remain a bit cooler.
* hard drive and ram are user replaceable on the mbp. not so in the air.
* air comes with onboard ssd (default build). mbp does not (upgradeable)
* mbp has firewire, thunderbolt, sdxc reader, superdrive, backlit keyboard
If (when) the air does get a hardware refresh to include the i5/i7 series processors, I do wonder where that leaves the 13" mbp model.
From a market segmentation standpoint, I don't see why two models that are so very similar continue to be offered. I can see why they still offer the non-pro 13" macbook (low end / entry level), and the larger pro models, but the 13" air and the 13" pro offer a potentially confusing choice to customers, and I would think this is a bad thing -- the "i can't decide so I will get neither" syndrome. Perhaps apple is waiting until they get thunderbolt onto the air before killing off the 13" pro.
In addition to segmentation, I would think it is advantageous to apple to prefer 'sealed' machines like the air, ipad, iphone, ipod, etc. These sealed machines put you on a faster upgrade cycle, because you have to refresh to replace or upgrade the ram/hard-drive. I would think this forced early upgrade cycle, along with the lean towards more utility computing, would move apple in this direction.
I am sure apple has historical metrics to support their offerings though, having had similar models offered in the past. So I am probably just 'talking out my ass' on the market segmentation issue....
note: edited for formatting