This is the integrated messaging app that you've been waiting for from Google. When you install it on your Android device, it will replace Google Talk completely. It took a few tries to sign in at first, but once we got in, it asked us to confirm our phone number, which was not for public viewing, but simply to make it easier for the app to handle video Hangout requests.
Who can contact you is controlled by your Google+ Circles, and there is some major brains behind this, because you may not even notice it if you don't open the settings. If you drill into the settings menu, you'll see options for controlling who can contact you, and your Circles are automatically filtered with Google magic. So, your Friends and Family Circles will be able to contact you without trouble, but more far removed Circles will only be able to request a Hangout with you. And, of course you can change these settings at will.
Hangouts is also available for Chrome, and when you install the Chrome app, it will live in a pop-up shade in the bottom right of your browser window. It's quite a lot like the original Google Talk app that existed for Chromebooks, if any of you out there got to play with an early CR48.
One odd thing is that Google Play is saying the app isn't compatible with the Nexus 7. No idea what that's about.
If you want to try out Hangouts, it's available for free in the Google Play Store, iTunes App Store, and Chrome Store.