I use an Air as my primary dev machine. I love it, but my allegiance has nothing to do with OS X. In fact, I would kill for a laptop with Air's form factor that was running Ubuntu as the primary OS.
http://www.engadget.com/2011/05/30/asus-outs-ux21-ultrathin-...
linux and other operating systems on the newest macbook air are currently being held up by a bug in the intel video driver (https://bugs.freedesktop.org/show_bug.cgi?id=39533) but all of the other hardware should work fine under ubuntu (http://ubuntuforums.org/showthread.php?t=1810275).