I've been testing various 3rd-party python code against 3.3b1, and see
  ValueError: level must be >= 0
exceptions where C code is using PyImport_ImportModuleEx.

PyImport_ImportModuleEx reads as
    63 #define PyImport_ImportModuleEx(n, g, l, f) \
    64     PyImport_ImportModuleLevel(n, g, l, f, -1)
within http://hg.python.org/cpython/file/aaa68dce117e/Include/import.h as of now (2012-08-09)

Within PyImport_ImportModuleLevel there's this check:
  1280      if (level < 0) {
  1281          PyErr_SetString(PyExc_ValueError, "level must be >= 0");
  1282          goto error;
  1283      }
which thus always fires.

So it would seem that currently any usage of PyImport_ImportModuleEx will fail.