If we could use a forward secure PGP, I think that could be worth breaking PGP backwards compatibility. Isn't the TextSecure protocol kind of that anyway? Why can't MailPile use that with an e-mail interface? It's forward secure, it's asynchronous and you can authenticate users. What more do we need?
Mailpile is an e-mail client, first, other things second. Maybe we'll support other protocols, such as TextSecure, at some point. I hope so, actually! :-)
Note that PFS generally applies to a communication channel, not the way a message is encrypted and signed for long-term storage. TextSecure decrypts messages and stores them using a different algorithm locally - there's no forward secrecy if someone steals your local TextSecure message store.
Sadly, the only such separation we have in the world of e-mail (at the moment) is the TLS wrapper around SMTP/IMAP/POP3, which can and should use PFS ciphers. But that doesn't help if the server itself is malicious. That's one of the reasons I'm keen to try and cut out the middle man when possible, and deliver "directly" from client to client. See our SMTorP discussion for one idea of how we might eventually do that: https://github.com/mailpile/Mailpile/wiki/SMTorP
