It isn't a hard thing to do, but you can't do it using any settings or options built in to the iPod or iTunes, but it can be done. Apparently, for $15 your can buy a little app that will do it for you.
There are several that are free as well.
smime.p7s
Description: S/MIME cryptographic signature
/* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
