When I first saw the MacBook Air commercial on TV it wasn’t the notebook I was paying attention to, but the music. It seems I wasn’t alone as this post from the PromoGuy.net site reveals the identity of the artist and the song as Yael Naim's New Soul. There’s also a link in the article to the album on iTunes if you’re into that.
Yael's Web site links to the following video, which I hope means it won’t get yanked due to copyright violation.
Edit on 2019-11-29: Updated links that broke in the years following the initial post in 2008.