I downloaded a free email certificate using Safari (could never find it when I used FF) and dragged it to the login item in Keychain Access. Under Certificates the email address certificate appeared but there were no keys associated with it and Mail gave me an error if I tried to send an e-mail with a digital signature. Removed the certificate and repeated the process and now the Sign and Encrypt icons don’t even appear in a new message.
I have the same problem with my iMac and 10.8.3.
Interestingly enough, on a 10.8.3 laptop the problem does not happen and I can see the keys within the certificate.