WhatsApp is still closed-source though, and I'm unsure as to the veracity of the claim of end-to-end encryption, I sure hope that's the case. In the Panorama documentary about Edward Snowden on the BBC, when discussing the death of Lee Rigby, a representative of Facebook says they're capable of decrypting messages (whether that applies to WhatsApp or just Facebook messenger is not clarified.)
https://www.youtube.com/watch?v=SjVxqlncS2I&t=46m15s (also see 49:00+)