In an effort to further streamline their MacBook Air, Apple is considering using carbon fiber as opposed to the aluminum currently used in the product's unibody design. Supposedly, this will bring the Air's weight down to under three pounds without decreasing the strength of the body.
This modification would be done to just the lower case. It seems a pre-production model of the MacBook Air made the rounds and the lower case was constructed from black carbon fiber. It was just as strong, but a bit lighter, down to 2.78 pounds. That's lighter than the Samsung X260, so that's pretty impressive.
This is currently only a rumor, but AppleInsider believes this carbon fiber bottom has been pushed deep into development, making a rerelease of the Air sometime next year entirely possible.