Yes, you can get a 256GB SSD in the 11" Air, either with the 1.6GHz Core i5 or the 1.8GHz Core i7. From the online store, click on the 11" Air with the 128GB SSD. When you get to the next screen you can select a 256GB SSD.
Note that some resellers (notably MacMall) have the 256GB model for less than Apple's prices, so you might check them out, as well.