I suspect it will only catch on once the big email providers find a way to make it transparent and cost-effective. And we start seeing verification icons in email clients above our messages. That way humans know it matters ;-)
The thing is, you can do that today with Apple's Mail.app, and have been able to do so for years. I sign my email using S/MIME, and while that wound up causing problems with a few outdated mail clients 5 or 10 years ago (sometimes, the fact that it was signed would make certain clients with poor MIME support show the body of the email as an attachment, which confused people), it doesn't cause much problem these days.
Cool. Still not as convenient as it could be, though. "If the intended recipient is outside the sender's Exchange environment or if the sender is not using an Exchange account, the recipient's certificate must be installed on the device."
What that means is by default any email you send would never appear "trusted" so... It's not a great marketing device. A green address bar does more to market SSL than its own advantages, sometimes. I'd argue that certificate trust -- even to say that the email address belongs to gmail.com, for instance, would do wonders to promote the technology.
Sadly, no support on Android, since apparently on Gmail everyone only emails within Google services and never for businesses? ;-) Microsoft should promote SMIME in its online Exchange offerings more, to compete with Gmail.
