There's a good blog on Mommywise about the pressure to breastfeed. I don't think the pressure to breastfeed contributes to postpartum depression, but I do think the pressure to breastfeed can be especially difficult for moms with postpartum depression.
I've written before about my struggle with breastfeeding because of a hormone patch designed to prevent postpartum depression. It lead to a low milk supply and therefore growth troubles for my little girl.
I still feel the urge to breastfeed, and my breasts still leak when I feel very emotional about my daughter. I feel bad for giving her a bottle, and I feel guilty in public for doing so. I try to remind myself that the bottle could be pumped breastmilk. No one knows what it is. That helps a bit to hold back the guilt.
I also try to remind myself that the reason I can't breastfeed - the medicine I take - is just as important. I'm more stable and present with her than without the medicine. I think it's better for us both, even if I had to give up breastfeeding early.