Facebook has confirmed to The Verge that the feature began rolling out to US users today, and requires no update through the App Store.
There's no video calling yet, so it's not a total, frontal assault on Skype, FaceTime, and other established VoIP services yet, but given Facebook's massive user base, that's a switch that can be flipped, and an attack that can begin at any time.
It worked fine when I tried it in Canada a while back, but no one hits a service like American hits a service. So, if you've taken Facebook Messenger calls for a spin, let me know how they sounded to you.
Source: The Verge