Using includeIn does seem like an element is becoming not visible, but actually the visible property of the element doesn't change, it is just no longer on the DisplayList.
You should be able to listen for elementAdd/elementRemove on the Group containing the element, for example:
<s:Button label="toggle" click="currentState=currentState=='state1' ?'state2':'state1'" />
<s:State name="state1" />
<s:State name="state2" />
<s:Group elementAdd="trace('added')" elementRemove="trace('removed');">
<s:Rect width="50" height="50" includeIn="state1">
<s:SolidColor color="red" />
Thanks for the helpful reply. It pointed me in the right direction.
It looks like "addedToStage" may be what I am looking for. (I was wanting to listen on the included component itself.)
The add and remove events of your child are trigged.