I'm currently looking for general feedback, but also specific feedback from App.net client developers on the feasibility and interest of implementing something like this into most (or all) clients. You can find me @ravisorg on App.net.
First you'll need to sign into App.net so we can create/retrieve your key pair. If you don't have one yet, this will prompt you to create one.
Encryption on this page is being performed by cryptico, although nothing here is really unique, you should be able to do it with any basic encryption library.
The original base16k blog post (written in JS). Demo no longer works, but it's archived here. Base16k has also been translated to C++ (which might help some of the native clients).