I see a couple ways you could introduce encryption securely.
1. At the very least, encrypt data being transfered using SSL, to prevent sniffing on WiFi and other public networks. Public key cryptography (like SSL) certainly can be secure, and does or doesn't require "cooperation" depending on your definition.
2. Your client could somehow integrate with something like TrueCrypt, but leave it up to the user to manage their keys.
1. At the very least, encrypt data being transfered using SSL, to prevent sniffing on WiFi and other public networks. Public key cryptography (like SSL) certainly can be secure, and does or doesn't require "cooperation" depending on your definition.
2. Your client could somehow integrate with something like TrueCrypt, but leave it up to the user to manage their keys.