Another indirect evidence that it is OK to charge your MBA with an 85W magsafe is that the Thunderbolt Display has a magsafe cable that Apple claims can charge up to 85W for either MBP and MBA, suggesting that the display is outfitted with an 85W magsafe that can be used for a lower watt MBA. If that is true, I would deduce that you can use an 85W magsafe adaptor to also charge a MBA. I've just got a MBP and MBA and swap the adaptors depending on convenience (i.e. I carry only the 85W around and use it to charge either laptop), and no ill-effect. Just my 2-cents.