Thanks to Senate gridlock, taxpayers engaged in estate planning will suffer whiplash over coming months as the federal estate tax disappears and reappears, possibly unexpectedly and retroactively. When the Senate refused to act Wednesday, it opened the door for the estate tax to disappear in two weeks--although no one knows for how long. Not only will the tax end, but the gift tax rate will fall to 35 percent. And, hardly noticed by most, only some assets inherited in 2010 will get “step-up” in basis.