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)