Consider this transcript from OS X 10.6:

>>> import locale
>>> locale.getlocale()
(None, None)
>>> locale.setlocale(locale.LC_ALL, _)
'C'
>>> locale.setlocale(locale.LC_ALL, 'en_US.UTF-8')
'en_US.UTF-8'
>>> locale.getlocale()
('en_US', 'UTF8')
>>> locale.setlocale(locale.LC_ALL, _)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/locale.py", line 494, in setlocale
    return _setlocale(category, locale)
locale.Error: unsupported locale setting
>>> 

OS X does not recognize 'en_US.UTF8', only 'en_US.UTF-8'.  However, when passed a tuple, the locale module tries to normalize the values.  It ends up turning the result of getlocale from ('en_US', 'UTF-8') into 'en_US.UTF8'.