So, I was looking at the adafruit forum
http://forums.adafruit.com/viewtopic.php?f=31&t=26873
There's a pic, shows a header that's labelled "power jumper"!
Here it is with the jumper (shunt) placed
Here it is with the jumper off
Is that the same board as yours? [Yours in your original pic and the version ref'd here look different to me.]
I like adafruit a lot, but their documentation isn't all it should be.