Earlier last year WhatsApp announced partnership with Open WhisperSystems to integrate the ratcheting forward secrecy protocol found in their app called TextSecure, into WhatsApp. The protocol is supposed to provide end-to-end encryption between WhatsApp clients. So far it has been implemented only in WhatsApp on Android, with the rest of platforms yet to come. The implementation however has already made it into unofficial WhatsApp libraries which allow developers to use WhatsApp service in their applications, starting with a python-library called yowsup, and the rest will follow. It’s worth mentioning that none of those libraries are supported nor approved by WhatsApp, so one has to wonder if WhatsApp is going to take some legal action (again) against them.
I would strongly advise against using any non-WhatsApp approved clients. Users of the unofficial WhatsApp client for Sailfish, Mitakuuluu, got banned from WhatsApp for using an unofficial client, after which Mitakuuluu’s developer ceased development. Know what you’re getting into!