A couple minor notes: I'm not really sure that a pen signifies "signed" or "signed by." I'd think it would mean "written by," or maybe "click here to write." I don't have a better icon idea, though.. maybe the seal-and-ribbons sometimes used to stand for "certificate"?
What if the From line had an additional verbal hint for signed mail: Subject: S/MIME mail compose From: Bob Lord <[EMAIL PROTECTED]> [signed] [etc.] Also, I know that you should never -rely- on color in UIs, but is it allowed to augment a UI with color? We could display the From line of signed messages in gold or blue or something. Are the soft white icons on the left of the banner (the pen-on-paper for compose, the back of the envelope for read) changeable? For unencrypted messages, we could display a postcard (pen-on-postcard) instead.
