MacBook Air for processing
My guess is that it depends on how much you are trying to do. If you are using it for casual shooting, lightoom and some filters, not an issue. If you want photoshop and multiple layers, it's going to be slower.
I can say that I have been using a Macbook Air C2D for 2 years, and for photographs it has been solid. For video, it's super slow in terms of export, but photos has been fine. I've been using Lightroom, and SilverEFX filters, and it has been a non issue.
I decided to order one of the new 13-inches so that video will work better, and I will have more horsepower if I decide to get a D800.