Dell is building own 15-inch Air, so why won't Apple?

The Wall Street Journal has it on good authority that computer maker Dell will mount on Tuesday another challenge at the MacBook Air’s dominance with the XPS 15z announcement, their newest ultra-thin, stylish notebook teased with the “The game is about to change” tagline. The 15-inch machine probably sports the aluminum unibody construction Apple pioneered with the Air. It’ll start out at $999 and Dell says it’s the thinnest notebook “on the planet”. This could be interesting for Apple which doesn’t yet have a 15-inch MacBook Air model in its notebook arsenal…