While I agree that a maxed out MBA enters the price range of a MBP, it still does not serve the same purpose and offer the same benefits.
First off, a 256GB SSD addition to a MBP would be a hefty price jump, not something you can really equate to the entry point MBP 15".
A lot of people have been saying that the new MBA is a secondary machine but I don't think it has to be. I use my MBA for everything. The only thing it physically cannot do is use CD media which I solve with a USB drive. How often do you really need that? If it is really that often, I think a Superdrive would be a fine compromise.
I'm an IT developer mostly using .NET technologies. I hate leaving Mac OS and I don't even like using BootCamp so I've been running Parallels 6 + Visual Studio 2010. I do all of my computing tasks on the MBA. For movies and music, I have a Time Capsule that contains all of that and I can even mount it remotely (via Internet) and listen to it wherever I have internet access (everywhere with phone tethering).
For 99% of people's needs, including a lot of power users, the MBA is more than enough. The Core 2 Duo is still a strong processor even if it is a bit 'old' and the Nvidia 320M is absolutely a beast.
So why not a crazy i7 MBP? First off, the iCore processors might as well be portable space heaters. They are amazingly fast, but honestly way too inefficient for their TDP. The current MBA is at 20 watts. The i7-920 can get up to 130 watts. That's going to be a lot of heat under heavy usage (and we all know how Flash is -amazing- at being generous on the CPU, /sarcasm).
I'd almost go as far as to say that the 256GB Air is not necessary, though the 4GB could be. For how cheap external storage is, why bother loading the Air up with everything that could be stored elsewhere (like movies/music). 128GB is more than enough for OS X, games, Window 7 partition, and all your documents.
The Air is about realizing that there is no need to have a huge computer with all this extraneous power. Why do you need 4 cores? Why do you need 3 GHZ? Why do you need 8-16GB RAM? Why do you need 1GB VRAM and 234829472348 shaders? You don't really. A modest process (C2D), strong mobile GPU (320M), decent RAM (2-4GB) and a solid state disk for performance with ample space.
So many people think that their GPU/CPU are the bottlenecks of their PC without realizing that a large majority of load times are based on I/O limits from the hard drive. You can't move stuff into memory faster than the disk can read it, even if your memory/CPU are a billion times faster. The load times you experience when you click Safari or in-game are all based on your hard disk. That's why the Air seems so fast, the solid state drive is simply amazing.
Take a sample of your daily usage on a computer. For the majority of the time, what are you doing? Are you seriously maxing it out or anywhere close? That's why the Air is beautiful, while it is powerful and able to do some serious tasks as well, it makes doing what you do most of the time that much easier and more responsive.