The MacBook Air is already a very light laptop, but since it"s introduction it has been beaten by it"s competitors, which have managed to get their notebooks even lighter than the already-light 1.36kg. According to AppleInsider, carbon fibre instead of aluminium will reduce the MacBook Air"s weight by 100 grams, which is exactly what Apple is planning to change in an upcoming refresh of their thinnest computer.
Carbon Fibre is extremely strong and very lightweight, but also very expensive. A full Carbon Fibre MacBook would most likely bump the price up to unacceptable levels, so Apple is considering only replacing the bottom part with carbon fibre, shedding the needed weight whilst also keeping the price acceptable. Currently, the bottom cover weighs in at 152 grams, with the top cover (with the Apple logo) weighing a heftier 260 grams.