Andrew Coulson has a nice piece at Cato on state versus federal education vouchers. He begins:
President Bush wants to help kids in failing public schools. That's good. He wants to do it by giving them federal vouchers to attend private schools. That's bad.
And the key paragraph is:
But if you study the subsequent 25 centuries of education history, you are driven to an inescapable conclusion: government funding of private schools brings with it government control – and the higher the level of government involved, the more serious the problem.
I am not sure I agree with Coulson that state-level tax credits are better than state-level vouchers; both require the government to define "education" and thus create the potential for excessive control. But that is a detail; Coulson's overall message is dead on.