HLdan has made some good points here.
I have personally seen 3 base model air in Singapore recently. That was the first time I had seen the rev b, and eventhough I've read so much about the line issues, I really didn't have a clue what it really is all about visually. Are the lines coarse and super visible? Are they the sorta band-like spectrum that fill the screen diagonally as some described? I didn't have a clue. The strange thing is even after I've seen the first air for 20 minutes, I still couldn't see the lines, until I compare the screen to the adjacent mbp's. Then the screen problem became apparent to me.
The thing with the mba screen is that, if you don't know what you are looking for, the chance of you finding it is low. HOWEVER that said, if you are like me who have had experience with good screens, a subtle sense of discomfort will slowly crawl into you after playing around with the air for some time - that something is wrong with the display, you just ain't sure what. It's a feeling of knowing something is just not right, but you just can't explain it in words.
But the problem becomes obvious when you compare a good screen beside it - in my case it was the mbp. Only then you will be able to discern the subtle differences. In the mba case, you will see thin, faint lines that are seemingly horizontal running across the screen. It's so subtle it makes one think it's the texture of the current background. What gives it away is that upon closer inspection, you realise the lines are not really horizontal, but slanting diagonally from left to right ever so slightly, and even so for different backgrounds.
A close range inspection on my screens at home and even that of the mbp's in the store, would only reveal the pixels. But for the air, you see not only the pixels, but a texture. It is this "texture" or what others refer to as "a band of electronic interference" that causes a subtle discomfort and deterioration in the screen quality. It's really hard to put this experience in words. It is something you have to see for yourself, and the best way to know what it is truely about is to have a good screen available for you to compare with.
As for me, after seeing 3 different mba from 3 different locations, I've yet to find one with a line-free screen. However that said, sometimes ignorance is bliss. If you have never seen a mbp or an ACD screen before, chances are you will appreciate the mba's better.