I missed this Bill Gates editorial from a couple of weeks back. In it, Gates argues against the public release of teacher "rankings"

I am a strong proponent of measuring teachers' effectiveness, and my foundation works with many schools to help make sure that such evaluations improve the overall quality of teaching. But publicly ranking teachers by name will not help them get better at their jobs or improve student learning. On the contrary, it will make it a lot harder to implement teacher evaluation systems that work... 

Many districts and states are trying to move toward better personnel systems for evaluation and improvement. Unfortunately, some education advocates in New York, Los Angeles and other cities are claiming that a good personnel system can be based on ranking teachers according to their "value-added rating" -- a measurement of their impact on students' test scores -- and publicizing the names and rankings online and in the media. But shaming poorly performing teachers doesn't fix the problem because it doesn't give them specific feedback. Value-added ratings are one important piece of a complete personnel system. 

But student test scores alone aren't a sensitive enough measure to gauge effective teaching, nor are they diagnostic enough to identify areas of improvement. Teaching is multifaceted, complex work. A reliable evaluation system must incorporate other measures of effectiveness, like students' feedback about their teachers and classroom observations by highly trained peer evaluators and principals.