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.
I don't think I get it; you don't have to set the board to USB power just to connect it to USB; if you're developing a battery-powered application, why don't you want to run off the battery (or other external power) ALL the time?