STARTTLS is supported on both IMAP/SMTP. Email from Google outgoing uses TLS encryption (at least to my mail server), email from my server to Google also uses TLS encryption.
IMAP is already TLS encrypted.
The biggest missing puzzle piece is making it simpler for people to get S/MIME set up (or PGP for that matter), and having it work with all major mail clients (yes, Mail.app is a major mail client).
Right now S/MIME still requires too many steps to get the lay person to set it up, same with PGP, we need something that is secure from the get-go with very minimal effort required on the users part.
The downside is that S/MIME and PGP don't really fit into the online world, no longer will it be simple to open the browser and go look at your email, you will be required to have your keys with you. Securing those keys becomes the second problem, one that has partially been solved with smart cards and other devices that will do signing/encrypting/decrypting on the card without giving up the private key... but loss is still an issue so key escrow becomes a big thing.
It is an interesting problem, with interesting challenges and I look forward to seeing how we as a group of technologists solve them. Once it becomes easy enough for grandma and grandpa to use secure encrypted communication it will become much harder to do wide-scale snooping on data.