The pkgutil.ImpImporter documentation says that if dirname is None, ImpImporter(dirname) should create a PEP 302 importer that searches the current sys.path, plus any modules that are frozen or built-in:

http://docs.python.org/dev/library/pkgutil.html#pkgutil.ImpImporter

However, the iter_modules() method of an ImpImporter instance doesn't search sys.path if dirname is None.  It returns a generator that always yields nothing.  For example--

Python 3.3.0b1 (default:5d43154d68a8, Jul  8 2012, 13:54:45) 
[GCC 4.2.1 Compatible Apple Clang 3.1 (tags/Apple/clang-318.0.58)] on darwin
>>> from pkgutil import ImpImporter
>>> importer = ImpImporter()
>>> list(importer.iter_modules())
[]

Strictly speaking, one could say the documentation only applies to the find_module() method since that's the only method covered by the PEP 302 API  However, iter_modules() is a public method.  So I think that either iter_modules() should be fixed, made private, or else the documentation clarified by saying that searching sys.path does not apply to iter_modules().

I'm pretty sure though that iter_modules() should be fixed.  This is because there are other functions in pkgutil that seem not to work because ImpImporter.iter_modules() behaves the way it does (specifically calling pkgutil.iter_modules() with path=None).