One of the most interesting questions. This hearkens back to my favorite Malcolm Gladwell piece (see "Tenure" post from August). There's a whole series on Marketplace, including interactive features. But I'm old-school. Here's the transcript of what I heard on the radio Thursday night.
http://marketplace.publicradio.org/display/web/2010/11/18/pm-improving-lousianas-schools-starting-with-teachers/