|It's been a while in the making, but this is well worth some attention.|
I've been lucky enough to be an early tester on CrypTweet - http://plexusproject.org/ - a public-key based method for encryption of Twitter DMs.
A base version is now available on the site linked above. It works nicely out of the box, but is kind of raw - minimal functional set, and all that.
For anyone interested in this kind of thing, for any number of reasons, may I (mis)quote that great Australian music commentator, Ian "Molly" Meldrum, and suggest you do yourself a favor by checking CrypTweet out.