The question is instead whether or not
others can assume that what you "said" is in fact your actual position on an
issue or whatever. In the case of a text/plain object there's little issue -
it nominally contains words you authored (unless otherwise indicated by the
context). However, as I said before, the act of forwarding and signing a
message should not be construed as endorsement of whatever the message
contains.
Sounds like what's needed here is the concept of Signed Attributes. A
Signed Attribute could be a comment, position, endorsement (or lack
thereof) associated with a particular message. The semantics of a
signed attribute could be done in such a way that the signature on the
whole (message plus attributes) would not be valid if either were
modified or not present.
See PKCS #7 (available via anonymous ftp from rsa.com) for more
details.
Cheers,
Steve Dusse