Not sure why everyone is so surprise, it was doom to happen sooner an later, and as I said it on many thread about the issue, I was almost certain that it was going to happen the moment they allow an official HotSpot app to the Appstore (in this case, it is build-in on the iOS, same diference).
I actually think is very "unlike" AT&T warning people about it and not just charging them (this could be due to the fact that they are just fishing now, and sending that notification to all high data rate users instead of actually going to check the traffic, which is very possible and it keep "casual" jailbreaks and modder from doing it, which is IMO what they want at the end)