From what I can tell, the consensus seems to be that you can safely use the 85w MacBook Pro power adapter with the MacBook Air. Here is an Apple article (HT2346) that seems to me to suggest that it's okay to use the 85w with the MBA:
http://support.apple.com/kb/HT2346
All that said, I have not used the 85w with the Air. Because most of my work is done at a few different desks, I ordered extra 85w adapters for my 17" MacBook Pro (one for the bag/travel, one for desk one, one for desk two). The thing that bothers me (with respect to using the 85w adapter with the Air) isn't the
wattage rating (85w) being higher than the Air adapter's 45w, or the
current rating being higher--but, specifically, the higher
voltage rating does concern me.
I understand wattage equals voltage times current. It has been my understanding that, in general (assuming tips match), that the use of higher current adapters with devices requiring lower current is okay, as long as the voltage matches. That is, I'm of the understanding that, in general, a 2-amp, 5-volt adapter should be fine with a device (for which the tip polarity and size match) requiring a 0.5-amp, 5 volt adapter. So, then, my specific issue here is that the decal on the outside of the 85w adapter does not show it auto-matching voltages down to 14.5v, only 18.5v and 16.5v.
I also read this:
http://alexking.org/blog/2008/10/03/macbook-air-battery-advice
I understand my concerns may be completely without merit--electricity and power adapters are not my areas of expertise. If my concerns are without merit, though, I'd appreciate someone explaining why.
I've already purchased three extra 45w adapters for my late-2010 MBA 13". Even then, as much as I move around, it'd be great to put one or two of the 85w adapters into service for the MBA so that I have plugs at the ready at
any of the spots I work at in two different buildings.