uhmm, AFAIK pep8 suggest UpperCamelCase for classes, under_score for packages, methods and functions[0]

btw: it's not a decorator :) but a camelCase->under_score renamer it's pretty easy to write

    import re
    
    def unCamel(obj, *maybe_camels):
        if not maybe_camels:
            maybe_camels = filter(lambda n: not n.startswith('_'), dir(obj))
    
        for maybe_camel in maybe_camels:
            underscore = re.sub(r'([^A-Z])([A-Z])([^A-Z])',
                lambda m: '%c_%c%c' % (m.group(1), m.group(2).lower(), m.group(3)),
                maybe_camel)
            if underscore != maybe_camel:
                setattr(obj, underscore, getattr(obj, maybe_camel))
    
    if __name__ == '__main__':
        class CamelCase(object):
            def multiWordMethod(self):
                print('called multiWordMethod')
        
        def multiWordFunction():
            print('called multiWordFunction')
    
        unCamel(CamelCase)    # magic
        import sys
        unCamel(sys.modules[__name__], 'multiWordFunction')    # magic
    
        CamelCase().multi_word_method()
        multi_word_function()

[0] http://www.python.org/dev/peps/pep-0008/