Well colour me surprised:

“On Tuesday, Whatsapp announced that it’s implementing end-to-end encryption, an upgrade to its privacy protections that makes it nearly impossible for anyone to read users’ messages—even the company itself. Whatsapp will integrate the open-source software Textsecure, created by privacy-focused non-profit Open Whisper Systems, which scrambles messages with a cryptographic key that only the user can access and never leaves his or her device. The result is practically uncrackable encryption for hundreds of millions of phones and tablets that have Whatsapp installed—by some measures the world’s largest-ever implementation of this standard of encryption in a messaging service.” Source.

It's only for Android so far but it's a very, very good start. Still a closed-source client, yes, so there is an large element of trust, but they certainly didn't have to go down this road - users don't seem to care.

Surely this devalues WhatsApp from their owner's (Facebook) point of view though? If they have implemented true end-to-end encryption, they won't be able to datamine all those juicy messages anymore...