Oh, one more thing:
I sign every email I send from my computer, thanks to my client's use of smartcards for employee badges. Windows and Outlook both make this really easy to do. Mac OS X, BSD Unix, and Linux? Not a great user experience there. And smartphones? I don't know if accessing a smartcard is even possible. I wouldn't mind putting certificates on my phone as long as accessing them would still require a PIN, but my client's enrollment process doesn't encompass smartphones yet, plus I sincerely doubt that they are as difficult to compromise as my smartcard.
All of the email servers I operate support STARTTLS. Transport-layer security is just as important to me as message integrity/confidentiality/authenticity. If I could get away with it, I would force all SMTP traffic over TLS, just like I already do with my web sites (even my intranet sites are HTTPS-only). As with my human correspondents, many of the MTAs out there don't support SMTP encryption.