Oh, you are right.  I even noted that bug in my PyCon talk, but immediately forgot about it :(  I do intend to fix it.

You can get it to work by explicitly passing the charset:

  >>> x = MIMEText('йцукен', _charset='utf8')
  >>> str(x)
'Content-Type: text/plain; charset="utf8"\nMIME-Version: 1.0\nContent-Transfer-Encoding: base64\n\n0LnRhtGD0LrQtdC9\n'

When I asked the use case, I meant specifically for calling encode_quopri, etc.  Given the above, do you need it anymore?