How pledged are the pledged delegates?

So, Obama and Clinton get to the convention. It's still a tie. They have a first vote, and it's still a tie. Now, as the wheeling and dealing for the superdelegates proceeds in back rooms, are the pledged delegates still pledged? Use to be, back when candidates were selected at the convention, that delegates were ONLY pledged for the first vote. After that first vote, the delegates could vote any way they liked. If that is still so, the wheeling and dealing for the second vote will be something to see.