I have the new MacBook Air 1.86/128GB SSD, and I too see very faint lines on my display. They are not completely horizontal - rather, they are pitched at about a -5 degree angle from left to right, and appear to be about 1 pixel in width, and 1 pixel apart.
What's interesting however, is that the lines change their position, relative to the actual screen image, depending on a slight change in viewing angle. In other words, these lines appear to "roll" over the actual screen image when the viewing angle changes back and forth.
This leads me to believe that they might be some sort of interference fringe caused by the interface between the LCD panel and the glossy surface, rather than a darkening (or lightening) of the display pixels themselves?
Regardless, I'm not going to exchange mine, given that many Airs seem to display this phenomenon, and my Air is pretty much perfect in every other way (no dead pixels, no dents, misaligned keys, flush lid when closed, etc.)