It drives me crazy when I hear radio commercials here in California asserting that a child attending preschool will reap untold benefits not only personally, but for society. One such commercial is playing on KFI at this very moment, narrated, I believe, by Adam Arkin.
As Joanne Jacobs notes above, studies have shown there are some benefits for disadvantaged children who attend preschool, but for other children the benefits are questionable, at best. I recently read an article about a study showing that full-time preschool could actually be damaging for some children.
Beyond the questionable value of preschool, there is the issue of cost. Joanne has a link to an article showing that in Quebec, Canada, their program is not costing the millions originally projected, but is now into the billions.