http://blogs.wsj.com/digits/2015/04/17/whatsapp-hits-800000-...
I haven't heard the final word on that. For all we know Facebook could pull the plug on the "in-development E2E encryption" in the next few months and act like nothing changed. Whatsapp has never made a public statement about it so you can't trust that it uses "decent security under the hood" right now.
When they make a public statement about rolling out E2E encryption to all of its clients and change their Privacy Policy to reflect that change (since they aren't supposed to "get your data" with E2E encryption anymore - therefore if the Privacy Policy doesn't reflect it, that means they aren't using E2E encryption) then we can at least have a reasonable expectation of them committing to it (even if in theory they could still bypass it and we would never know, because it's a closed source app, but at least if we do find out about it, we can hold them to it - right now we can't hold them to anything because they've never said anything about the E2E encryption themselves).