I've been working for the past few days on a patch for SMF 1.1.19 to add 2 factor authentication (whenever I have free time).  I plan to add it to both the login and password recovery (this will be configurable per user) and plan to support Google Authenticator and signed PGP messages.

I don't have a lot of free time to work on it but I hope to have something to send to Theymos soon so I can claim that bounty.