What influences teachers' modifications of curriculum?

We worked with three cohorts of secondary science teachers in a project to study how teachers adapt plans and curriculum.  We found few "responsive modifications," meaning modifications that responded to the substance of students' thinking, which led us to shift our focus to understand the dynamics of teachers' attention in class.  We are currently (09-10) funded by a supplement to develop case studies of secondary student thinking, to use in teacher education.   Dan Levin, now at American University, is taking the lead in that development.