When run under Python 2.3 and fed a unix mbox file
on stdin, this script prints the lengths of the messages
in the file and their subjects on stdout:

import sys
import mailbox
import email.Parser

for msg in mailbox.PortableUnixMailbox(sys.stdin, 
email.Parser.Parser().parse):
    print len(str(msg)), msg["subject"]

If run using Python from CVS it prints out "38" as the
length of each message and "None" as the subject.
Something clearly changed between 2.3 and 2.4.
I've not had a chance to dig into the problem yet.
Am I using PortableUnixMailbox or email.Parser.Parser
in some unsupported and now broken way?

Assigning to Barry since he's the obvious email guru,
but anyone can feel free to jump in here.  This would
seem to be a fairly serious bug.

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Further investigation shows this simpler script demonstrates
the problem:

import sys
import mailbox

for msg in mailbox.PortableUnixMailbox(sys.stdin):
    print len(str(msg))

Looking at mailbox.py it appears v 1.42 is the culprit. 
Reverting to 1.41
solves the problem.  Reassigning to Andrew since he checked
in that
version.

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That was my patch, and it was my fault. I forgot to stick
the return calls in the read() and readline() functions. A
patch is unnecessary, I presume? 

This must be my dumbest mistake yet. 

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Perhaps it's your dumbest mistake yet, but I'm sure I've made dumber. 
;-)

Checked in mailbox.py as v1.43. Also added simple test case to 
test_mailbox.py v1.10.