I've been looking into my problems with forwarding messages some more,
and I believe I've found two separate bugs in Gnus.  If you're not
convinced, could you let me know what else you'd like to me to look
into?

I'm including my original message below.  This time I edited and
resent it as an 8bit encoded message.  If this works the same sending
to the Gnus mailing list as it did to myself, the attached message
will look fine.

Bug 1:  If I press C-d on this message and view the attachment, the
special characters "=" and "ê" show up as their quoted-printable
equivalents, even though the attachment looks fine as I compose the
message, and the entire message looks correct.  A variation on this
would be why my original forwarded message looked wrong in the
complete view, but OK given Kai's suggestion to use C-d.

Bug 2:  When viewing a quoted-printable encoded message, forward it
with S o m.  The message in the buffer includes a header line

    Content-Transfer-Encoding: quoted-printable

but the actual data in the message has been decoded.  When Gnus, Lotus
Notes, mutt and probably other mailers (though not the mailing list
archives) receive the message, they decode it for quoted-printable
again, resulting in the error I had.

If I manually edit the message in the *mail* buffer and change the =
to =3D and the ê to =EA, the message is encoded and received properly.