I don't see any way that this could work using PGP, but I confess I
don't know enough about S/MIME to know if it could be done using it or not.
The one list manager that I know of that actually does S/MIME, Sympa,
sends a separate copy of the message to each recipient encrypted to
his key. I think it's more likely because it's easier to code than
due to security concerns since the list management software usually
lets each subscriber look at the subscription list and archives.
With respect to Doug Royer's question, I believe one normally
configures an encrypted list to accept only encrypted submissions. At
some level you have to trust the list software not to lie too badly
about what it's doing.
R's,
John