Add me to the list of those with this problem, although it is only just now manifesting after about six or seven months of otherwise trouble-free use. The "device not supported" message is only just now cropping up occasionally, and I've been ignoring it because the keyboard has been working anyway, but today it came up and the keyboard didn't work and I had to disconnect and reconnect it a few times to get it recognized again. That's when I came here to see if it was a known problem and found it indeed is.
Obviously, it's a hardware problem, since it takes a while to start happening and I'm glad to see there is an extended replacement program too (I bought mine as an "open box" discounted item on eBay, so I don't know if that disqualifies it in Apple's eyes or not. How do we register an accessory with Apple, the way we do with a major item that contains a serial number?) but I'd rather it not get to the point it is so unusable that I'm forced to replace it.
So far, this is just a sporadic, random annoyance. But I've wondered about the long-term effects of constantly flexing the keyboard's material and what that's doing to the integrated circuits and whatever else is inside there that makes the whole thing work. I like Apple's clean design of the keyboard, but I've been worried about durability when noticing the flex marks and frayed edges where it is being constantly bent and bent back.