Signal for Android is already available, kinda...

It's split into TextSecure for the messaging portion and RedPhone for the call portion[0].

[0]https://whispersystems.org/

There are some compatibility problems between Signal and RedPhone. I can't seem to get the two to work together for international calling. Googling "Signal can't connect to RedPhone" shows that I'm not alone in this. Guess I'll have to wait for the Android version of Signal.

For desktop- https://github.com/WhisperSystems/TextSecure-Browser

For those who might clone, build, and use without actually reading the readme, take note of this warning by the project maintainers:

"warning: This project is still in the prototype phase. It contains many bugs and lacks many features."