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I haven't heard much about this, but because Apple produced new cases for the Air 2, it seems like many, if not all, of the Air cases won't work on the 2. I'm not sure, though.
 
From Apple:

Which Smart Cover or Smart Case should I use with my iPad?

The iPad Air Smart Cover is compatible with both iPad Air 2 and iPad Air. The iPad Air 2 Smart Case fits only iPad Air 2. The iPad Air Smart Case fits only iPad Air. The iPad mini Smart Cover and the iPad mini Smart Case work with all iPad mini models.

http://store.apple.com/us/buy-ipad/ipad-air-2 (Scroll to the very bottom for the FAQ)
 
My air case fits. The only problem is that the camera on the air 2 in not in the same location as the Air. You'll need a new case if you wanted to use the rear camera.
 
I can confirm that my Logitech ultrathin keyboard cover fits okay. The only minor problem is that the iPad, when sitting in the groove you put it in to use the thing, tilts backwards a lot more than intended giving a totally odd viewing angle. Not unusable, but, clearly, not intended.

Still, this will work fine until an updated version is produced and when you hook it up for cover use, you can't discern any issue with the fit.

So basically it seems like accessories that work just like Apple's smart cover (not case) should be okay, mostly.
 
I have the fabricskin folio for the iPad air and it doesn't work with the 2. It fits fine, but the magnets don't line up the same way to turn on the keyboard. So, since the keyboard won't even turn on, there's no way to use it with the 2. #
 
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