This was an intentional change, but I'm having second thoughts about it.  I think I need to make it a deprecation warning in 3.4.

Note that it doesn't actually do anything useful in 3.3:

Python 3.3.2 (default, Dec  9 2013, 11:44:21) 
[GCC 4.7.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import email.parser
>>> meta = """
... Header: ☃
... """
>>> m = email.parser.Parser().parsestr(meta)
>>> str(m)
'\nHeader: ☃\n'
>>> import email.generator
>>> import io
>>> s = io.BytesIO()
>>> g = email.generator.BytesGenerator(s)
>>> g.flatten(m)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.3/email/generator.py", line 112, in flatten
    self._write(msg)
  File "/usr/lib/python3.3/email/generator.py", line 177, in _write
    self._dispatch(msg)
  File "/usr/lib/python3.3/email/generator.py", line 203, in _dispatch
    meth(msg)
  File "/usr/lib/python3.3/email/generator.py", line 421, in _handle_text
    super(BytesGenerator,self)._handle_text(msg)
  File "/usr/lib/python3.3/email/generator.py", line 233, in _handle_text
    self._write_lines(payload)
  File "/usr/lib/python3.3/email/generator.py", line 158, in _write_lines
    self.write(laststripped)
  File "/usr/lib/python3.3/email/generator.py", line 395, in write
    self._fp.write(s.encode('ascii', 'surrogateescape'))
UnicodeEncodeError: 'ascii' codec can't encode character '\u2603' in position 8: ordinal not in range(128)

That is, if you pretend the message is a string, it will happily output
it as a string, including perhaps your writing the output to a file as
utf-8...but it will *NOT* be a valid email message, since it will have
non-ascii data in it with no specified CTE.