^ that.
Apple is trying to make SSD's more mainstream by making it the only storage option in MBA's. That way they hope it doesn't have to take long before they can do the same for their MB's and MBP's, so they'll get even more room for bigger batteries and discrete graphics.
The previous Air's hard drive was only a 1.8" type, it's not hard to exceed the performance of that type. Seeing the price of large capacity SSDs, I'm willing to hold off another year before I dive in.