Well, one theory is that postpartum depression is associated with depletion of essential fatty acids from the mother's brain to help develop the baby's brain:
http://www.obgyn.net/newsrx/womens_heal ... 101-19.asp
According to that article, "A mother's brain, which is 60% fat, can literally shrink 3% each time she has a child unless she intakes essential fats sufficient for the needs of herself and her baby".
The main fatty acids that tend to be missing from modern diets are omega 3 fats; the ideal ratio of omega 3s to omega 6s is about 1:2, and most Americans' diet has a ratio closer to 1:20. Good sources are fatty fish - sardines are good, since they are also low in mercury - and "omega 3" eggs from hens fed a diet high in flax seed - check for eggs that advertise at least 200mg of omega 3 per egg. Regular eggs are not good and corn oil is also particularly bad in this regard.
I cook for my wife and myself, and switched us to a diet high in omega 3s for her pregnancy. She had the opposite of postpartum depression - after having the baby, she really felt happy for the first time in years. Partly that was finally getting the baby we had been trying for for so long, but the diet may well have had something to do with it too.
For what it's worth, we follow the paleo diet. In addition to keeping my wife and our baby happy, it's also helped my wife steadily drop the weight gained during IVF treatment and pregnancy. If you're interested, you can find more information at http://cavemanforum.com/