Encrypting tweets using public keys and MIME64 seems easy enough, eg: http://www.c0t0d0s0.org/archives/5330-Encrypted-direct-messages-in-Twitter.html
Has any client implemented something like this? -- We're just a Bunch Of Regular Guys, a collective group that's trying to understand and assimilate technology. We feel that resistance to new ideas and technology is unwise and ultimately futile.