Sorry for the double post, but the problem just solved itself today. The app told it needed to update, I downloaded the update on the appstore (iOS) and during the first launch of the updated app, I got a prompt telling me the device needed an update.
I launched it, it worked for one my two Threes (the speaker made a "bip" sound), I got an error message for the other one. I tried again, and it finally worked.
Since then, no more "update" message in the app, and both speakers seem to work like a charm.
To sum it up, it seems the new version of the app allows to update the devices "over the air" and without USB keys, and most importantly, works fine. Hope it will stay that way !
