I wish people would just let the PGP/GPG dream go. S/MIME is supported by pretty much every serious mail client out there, including mobile ones such as iOS and BlackBerry. The certificates cost next to nothing and most clients automate signing/encrypting decisions. I don't understand why this is not used more broadly. Who doesn't want a cool 'signed' seal next to their email?
Everyone doesn't want a cool signed seal next to their email. That's backwards. I have had colleagues do S/MIME, and then they stopped using it because at the time, Android didn't support S/MIME. As a result I'd get some emails from them with a signed seal, and some without. Did I ever suspect that I wasn't receiving legitimate email from them? Nope. If you are signing with S/MIME and then stop, what happens is that everyone receiving your email thinks "Oh thank God, that stupid ribbon thingie has stopped showing up on InterBigs emails".
Doesn't S/MIME kind of depend up on you using only one email client? The client with the certificate. Who uses just one email client these days?
I think for S/MIME to work, no one would see the cool S/MIME signed seal from signed emails. What should happen is that the client should notice that after receiving a boatload of S/MIME emails from a contact, and then one shows up without it, the client should flag that message as suspicious.