Hi, I want to suggest a tiny change to the PWR_SEL jumper.
It's easy to modify an existing board, even easier on a kit.While it doesn't matter much in the bare configuration, once you stack shields onto the Arduino, it becomes a pain to change from USB power to external and vice versa. Simple solution: replace the jumper with a micro switch. This adds minimally to the cost, but improves usability by so much.
You can see the modification here:
Hope this may be useful to some.
Possibly Corax,
How often do you want to change power source?
The Geek.