Note that after my change was made, @sebmarkbage made a change to cache the result of getDefaultProps across instances so now you can't refer to the current instance as this there. The proper solution is to do:

<p onMouseOver={this.handleMouseOver}>...</p>

handleMouseOver: function(e) {
  if (this.props.onMouseOver) {
    return this.props.onMouseOver(e);
  }
  // default behavior here
}

I don't understand. Can you show me an example involving getDefaultProps & transferPropsTo, has that changed in any way?

In getDefaultProps you can't access this any more; it is called only once and refers to the class instead of the new instance being created. That is, if you have

var A = React.createClass({
  getDefaultProps: function() {
    console.log("getting default props!");
    return {width: 120};
  }
});
React.renderComponent(<A />, node1);
React.renderComponent(<A />, node2);

then the default props will be looked up only once and shared, instead of twice as they were before. There's no way to do what your original example is aiming for using only transferPropsTo. You have to specify the handler explicitly when rendering.

Hmmm ok. That's kind of a breaking change. I've seen a lot of people use this pattern to share properties and behaviour between components. How would you recommend architecting the first code example if I wanted to share the Highlight mixin between two different components?

I could as you say specify the handlers on the actual components, but in this case it's not so DRY because i have like 20-30 components sharing the same behaviour. Adding a new event handler to that mixin will require me to edit all these components instead of just the mixin.

It is a breaking change. That's why I'm telling you here and why it'll be mentioned prominently in the release notes. Mixins are generally not the preferred way to share functionality. I don't know you exact use case but you might be able to do something similar with composition like this:

var Highlight = React.createClass({
  highlight: function() {
  },
  unhighlight: function() {
  },
  render: function() {
    return (
      <div onMouseOver={this.highlight} onMouseOut={this.unhighlight}>
        {this.props.children}
      </div>
    );
  }
});

var Paragraph = React.createClass({
  render: function() {
    return (
      <Highlight>
        <p>Hello</p>
      </Highlight>
    );
  }
});