It is not obvious whether encoding of an unicode argument will happen or
not:

« [...] No guessing or encoding is performed on the text data.

Changed in version 2.4: The previously deprecated _encoding argument has
been removed. Encoding happens implicitly based on the _charset argument. »

http://docs.python.org/library/email.mime.html#email.mime.text.MIMEText

Apparently it doesn't:

>>> message = MIMEText(u"héhé", _charset="utf-8")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.6/email/mime/text.py", line 30, in __init__
    self.set_payload(_text, _charset)
  File "/usr/lib64/python2.6/email/message.py", line 224, in set_payload
    self.set_charset(charset)
  File "/usr/lib64/python2.6/email/message.py", line 266, in set_charset
    self._payload = charset.body_encode(self._payload)
  File "/usr/lib64/python2.6/email/charset.py", line 387, in body_encode
    return email.base64mime.body_encode(s)
  File "/usr/lib64/python2.6/email/base64mime.py", line 147, in encode
    enc = b2a_base64(s[i:i + max_unencoded])
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in
position 1: ordinal not in range(128)

UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)

The lack of unicode encoding support should be mentioned explicitly.
Attaching a doc patch.

This is not quite a duplicate of issue 1368247, but in the patch for that issue the doc changes should clarify the ambiguity.