I am a regular reader of the (free) ASCD SmartBrief. Quoting from the 9/9/2010 (http://www.smartbrief.com/servlet/ArchiveServlet?issueid=539D4C89-3ABF-4700-9FA9-4E814961C5C2&lmid=archives) issue:
Philadelphia high school implements mastery-learning model
Class is back in session in Philadelphia, where educators are working to raise achievement at six schools designated by Superintendent Arlene Ackerman as Promise Academies. One high school is adopting the mastery-learning model of instruction. The approach uses pass-fail ratings instead of letter grades and allows students to progress through subjects at their own pace.