Okay, so this is just about the coolest optical illusion I've ever seen. Especially the second part about the pink dots disappearing all together.
Apparently the visual cortex only cares about differences and the pink dots are fuzzy enough (especially when you are focused on something else) that they seem constant even though the eye is doing its usual fast little movements (saccades). The green appears because a reduction in pink and a gain in green are the same thing.
Jenni pointed out to me that
if you fixate somewhere outside the circle, it still works, but if you move your focus to another point outside the circle, you can see the pink again. You don't have to look at the pink dots again to make them reappear. You just have to move them to a different point on the retina.