from Active Birth Center:
"Pigmentation changes are common in pregnancy due to hormones.
These may include darkening of the nipples and surrounding skin. A dark line called the 'linea negra' may appear down the centre of the abdomen around the third month. Birthmarks, moles and freckles may temporarily darken or increase in number or size.
Occasionally the face may darken giving the impression of a sun tan, especially around the mouth. This is called chloasma or the 'mask of pregnancy.' Pigment changes usually disappear after pregnancy."
from reading a few more web sites, it doesn't appear to be linked to anything in particular. light-skinned women are less likely to have it develop; some darker women will have it stay even after the baby is born.
thinking along the lines of evolutionary biology, i wonder if it was some sort of way to increase ability to camoflauge ourselves during a vulnerable period? however, talking to my population biologist hubby, he says that it's more likely a response to the hormones, rather than a survival trait or something else... more of a side effect than a cause.
i guess it's just one of those funky pregnancy things!