The Federal Tax rule is in the tax law but each state has tax laws on 529's held within the state stince 529's are state managed items.
Per articles I have read the states are being a bit slow to implement the 10k tax rule so if you withdraw from a state that has not made the change you could be penalty by the state itself not the Fed.
Example from Washington Post article: "states, such as Iowa, Maine and Nebraska have warned that they may have to introduce 529 legislation before residents can be assured of favorable tax treatment."
HOWEVER, that same article says that SavingforCollege has a list available (which I didn't find initially). It included the two states I list. The article talking about this is here: