There is no event that I know of that will fire when the enabled attribute is changed. Your best course of action would likely be to override the setter function for the component that updates the enabled property, and fire off a custom event. Or, if you control the part of the component that is taking action(a button for instance) you can use a wrapper function to do your event functionality, and then update the component. For example:
private function setEnabled(enabled:Boolean):void
// your logic
customComponentName.enabled = enabled;
<mx:Button label="Disable" click="setEnabled(false)" />
<mx:Button label="Enable" click="setEnabled(true)" />
That is not tested -- it's just for example purposes.