React 0.14.8 correct sets the defaultValue for `<select>` when using `<optgroup>`:
https://jsfiddle.net/6v8cnowo/

However, 15.0 RC2 defaults to the first `<option>`:
https://jsfiddle.net/p2ctcgwj/1/

I'm not very familiar with the React codebase, but I suspect it has something to do with a change made in 1dca72e, specifically this line: https://github.com/facebook/react/blame/master/src/renderers/dom/client/wrappers/ReactDOMOption.js#L35

expect(node.options[1].selected).toBe(true);  
    expect(node.options[2].selected).toBe(true);  
    
    ReactDOM.render(
      <select multiple={true} defaultValue={['monkey']}>{options}</select>,
      container
    );
    expect(node.options[0].selected).toBe(false);  
    expect(node.options[1].selected).toBe(true);  
    expect(node.options[2].selected).toBe(true);  
  });
  it('should allow setting `value`', function() {
    var stub =
      <select value="giraffe" onChange={noop}>
        <option value="monkey">A monkey!</option>
        <option value="giraffe">A giraffe!</option>
        <option value="gorilla">A gorilla!</option>
      </select>;
    var options = stub.props.children;
    var container = document.createElement('div');
    stub = ReactDOM.render(stub, container);
    var node = ReactDOM.findDOMNode(stub);
    expect(node.value).toBe('giraffe');
    
    ReactDOM.render(
      <select value="gorilla" onChange={noop}>{options}</select>,
      container
    );
    expect(node.value).toEqual('gorilla');
  });