They're called Kale and they're a type of cabbage, a very pretty type of cabbage in fact. So obviously, they're edible. After staring at them all night we finally had a nibble and low and behold, they taste like cabbage. Astounding.
In Japan, Kale juice is known as 'aojiru' and is used as a supplement, I suppose similar to wheat grass juice or aloe juice. It contains all sorts of vitamins (the A's, the B's and the E's) and oh my god, there is a blog devoted to recipes featuring it, called (surprisingly) I Heart Kale.