I was just thinking about this the other day as well. The idea is to create a component that has the board outline and header placement for a shield for each version of Arduino modules. This would make it easy (less difficult?) to create a new shield project. You could spend time on the shield function and not have to worry so much about getting the shield interconnects right.
Your ProMini looks mostly good. I downloaded your component and added it to the ProMini eagle board definition downloaded from Sparkfun. The headers line up correctly, but the component board outline appears to be just slightly larger than the ProMini outline. If that was your intent then you succeeded. Good luck with your board.
I plan to do one of these for the Mega Pro module, unless someone has aready done one. Do folks who have more experience think such component definitions of the board interconnects is useful?
I guess the other way to do this is to start from the Arduino eagle definition, duplicate it for the new design and delete the unnecessary components, then add components for the new design.
What do other folks do?
Does anyone have any other suggestions about how to do this?