When using getAttributeNS, attributes with no namespace should be considered as having the default namespace for that scope.  See examples in http://www.w3.org/TR/REC-xml-names/#defaulting.  Python's xml.dom.minidom will always set the namespace to None for attributes that have no namespace prefix.

I've attached a test program to illustrate this issue in action.  The output I get is:
[((None, u'attr'), u'value1')]
[(('http://www.w3.org/2000/xmlns/', 'xmlns'), u'http://path/to/ns2#'), ((None, u'attr'), u'value2')]
[((u'http://path/to/ns2#', u'attr'), u'value3')]
Successfully got child3 attr value

In the first two cases the namespaceURI is None, but it should be set to the default namespace specified in the root element.  I believe this problem occurs with all *NS functions.  Not tested in 3.x.