Steve Landsburg notes that the economic incidence of a tax is separate from its legal incidence. So, if you try to tax, say, savings accounts of the rich, you can't really say upon whom the real burden of the tax will fall. I think that's right.
But then it can't also be right to complain that high marginal rates discourage productivity, because that argument assumes the tax burden stays right where one puts it, doesn't it? Or have I missed something?