That's funny. I was wondering the same thing when I first opened up my MBA! I asked a Genius at the Apple Store who proceeded to tell me that he had wondered the same thing too, and asked his superior who told him that is was designed for use with the MacBook Air Superdrive, which costs an astounding $79 and is basically a slick looking external DVD writer. Because it's made by Apple it doesn't have any buttons on it, including an eject button. Most new/casual Mac users won't know the drag to eject or context-menu eject methods, so in order to make the drive streamlined with the Mac, they put the eject key on the MacBook Air itself.
I guess Apple anticipates a wide user base for the new Superdrive...