Universally available, non-compulsory preschool, subsidized based on means-testing, is worth doing purely because it would help low- and middle-income families deal with the difficulty of raising a family while both parents hold down jobs. It would also be a great form of economic stimulus, since both the recipients and the providers of the service are likely to be low- and middle-income people. Even if it had no lasting educational benefits, it would be a good idea.
It also has a lot of potential to give low-income kids a chance to become well-educated adults on a more equal basis with kids who come from money. That’s all to the good.
But please: let’s not create another layer of test-driven obedience schools. Unfortunately, that’s what we have every reason to expect from the people who brought us No Child Left Behind and Race to the Top. The people running education policy today are among the very last people I would entrust a three-year-old to. I wish I thought otherwise.