I agree with those who think it unlikely that Apple would ever offer a 15 inch MBA. The MBA is a niche produce, an ultralight and extremely thin laptop. While some of that could be preserved even in a 15 inch machine, some of the MBA's current thinness and a lot of its lightweight would have to be given up. Thus, I don't think it will happen, either.
On a side note, despite swearing I wouldn't, I went to the Apple Store to scope out the iPad. Ha! I got there at 9:05 am and the store was full, with still more customers lined up outside the door as far as the eye could see. The line was so long I didn't bother to actually ascertain where it ended. It was the wildest scene I had seen since the original iPhone came out. Steve Jobs may have had the right idea, at that.