AppleInsider is today reporting that Apple has been looking into replacing the aluminium enclosure on the MacBook Air with a carbon fibre case. Apple is apparently happy with the "thinnest" of the MacBook Air but wishes to trim the weight to below 3 pounds (1.3KG).
AppleInsider adds that Apple will likely keep the upper aluminium enclosure and only replace the lower enclosure with carbon fibre. Finally the rumour site believes the change could come as soon as next year.