Joanne Jacobs cites to a study which reports that the neediest kids are not in the top quality pre-schools.
Forgive me for being optuse but DUH!!!.
Look top quality preschools cost money (and I know this for a fact). By definition low-income, poorly educated mothers don't have the income (hense the term "low-income") to pay for a high quality preschool. Those mothers who do send their children to Head Start programs get some benefit, but top quality pre-schools, which are by necessity private businesses for the most part, come with a top quality price-tag.
You didn't need a big massive study to tell you that.