And so it happens, the commit we're waiting since about 6 months it's now pushed to master, React Native for Android is here.
Starting today you can really "learn once, run anywhere".

react-native (v0.11.0) on GitHub

This is an early release and there are several things that are known
not to work if you're porting your iOS app to Android.

See the Known Issues guide on the website.

We will work with the community to reach platform parity with iOS.

// iOS

var React = require("react-native");
var { TabBarIOS, NavigatorIOS } = React;

var App = React.createClass({
  render: function () {
    return (
      <TabBarIOS>
        <TabBarIOS.Item title="React Native" selected={true}>
          <NavigatorIOS initialRoute={{ title: "React Native" }} />
        </TabBarIOS.Item>
      </TabBarIOS>
    );
  },
});

// Android

var React = require("react-native");
var { DrawerLayoutAndroid, ProgressBarAndroid } = React;

var App = React.createClass({
  render: function () {
    return (
      <DrawerLayoutAndroid
        renderNavigationView={() => <Text>React Native</Text>}
      >
        <ProgressBarAndroid />
      </DrawerLayoutAndroid>
    );
  },
});

The countdown is over! We're super excited to launch @reactjs native for Android. https://t.co/dWeMC0JYbV #androiddev

— Facebook Open Source (@fbOpenSource) September 14, 2015

Just pushed React Native Android to github at @scale :) Please keep in mind it's still very early! https://t.co/uJn94lAKSh

— React (@reactjs) September 14, 2015

React Native for Android: How we built the first cross-platform React Native app

Earlier this year, we introduced React Native for iOS. React Native brings what developers are used to from React on the web — declarative self-contained UI components and fast development cycles — to the mobile platform, while retaining the speed, fidelity, and feel of native applications. Today, we're happy to release React Native for Android.

At Facebook we've been using React Native in production for over a year now. Almost exactly a year ago, our team set out to develop the Ads Manager app. Our goal was to create a new app to let the millions of people who advertise on Facebook manage their accounts and create new ads on the go. It ended up being not only Facebook's first fully React Native app but also the first cross-platform one. In this post, we'd like to share with you how we built this app, how React Native enabled us to move faster, and the lessons we learned.