Politico: Nullification is unconstitutional.
I really hate to defend Politico, but he never mentioned nullification. It's a pure tenth amendment argument here. "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
If the individual mandate does not fall within the "powers not delegated to the United States by the Constitution" then since it is not "prohibited by it to the States" then it wold be unconstitutional for the Feds to implement, but perfectly legal for the States to do so (subject to any restrictions imposed by their own constitutions).