Like many others I can't see the benefit of going Apple over the many other slimmer or beefier Magsafe battery packs out there on the market, and no doubt some newer ones will be made to better match the profile of the Air.
Only thing I'm wondering about is whethere there are any iOS specific smarts to the Magsafe battery pack. Other than Mactgagonist's comment above about the battery widget, I wonder if there's any connection to the phone which enables it to offer some other benefits eg charge speed varies depending on other phone specific or environmental factors.