So, how much should a teacher know? The following quote from the blog post touches this:
It seems obvious that teachers must have knowledge of the subject matter they will actually teach. But how much more knowledge should a teacher have than what she or he is seeking to assist students in learning? The case of secondary school mathematics is instructive. Is it enough for a high school trigonometry teacher to know trigonometry cold – but not, say, real analysis, or ordinary differential equations?
This issue was exactly the one that was raised in the LMT project (Learning Mathematics for Teaching) at University of Michigan. This was also the main issue in an article written by Heather Hill, Deborah Ball and Stephen Schilling in the last issue of Journal for Research in Mathematics Education. (The LMT team has also written several other scientific articles about the issue.)