The keyword-only idea is a backwards compatibility hack we discussed at the PyCon US sprints because ImportError currently accepts an arbitrary number of arguments:

>>> raise ImportError(1, 2, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: (1, 2, 3)

We don't really want to be claiming that the module name is some strange value due to existing code that passes multiple arguments, hence the suggestion to make this a keyword-only argument.

Regarding import.c, I think Brian misinterpreted Brett's last message. import.c absolutely *should* be modified by this patch, since it's still the default import implementation for the moment. Brett's comment was just to say that *he* wasn't going to do that part, since his aim with the importlib bootstrapping exercise is to nuke most of import.c from orbit :)