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.