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Passing both a value for i and decode=True to email.message.Message.get_payload(), when self is a multipart, returns None when it should return a string. The reason is that an is_multipart() test is done on self when it should instead be done on the selected payload part. Here's a patch that fixes this:
--- /usr/lib64/python2.7/email/message.py	2011年10月26日 18:40:36.000000000 -0700
+++ /home/bobg/tmp/message.py	2012年05月07日 06:34:28.579401481 -0700
@@ -192,9 +192,9 @@
 else:
 payload = self._payload[i]
 if decode:
- if self.is_multipart():
+ if payload.is_multipart():
 return None
- cte = self.get('content-transfer-encoding', '').lower()
+ cte = payload.get('content-transfer-encoding', '').lower()
 if cte == 'quoted-printable':
 return utils._qdecode(payload)
 elif cte == 'base64':

Incidentally, a workaround is:
msg.get_payload(n).get_payload(decode=True)

Thanks for the report and patch suggestion, but...
This is actually the way it is designed to work. get_payload(i) returns the ith element of a multipart payload. Your workaround is in fact the correct way to do this operation. decode=True is documented to return None if is_multipart is True.
You will note that if decode=False, you get back the Message sub-object, not a string. Since decoding a Message object (as opposed to its payload) is not a meaningful operation, None is returned for decode=True. Arguably we should raise a TypeError or ValueError instead, but we don't for historical reasons.