I recall hearing Dr. Miriam Labbok citing many studies (but not all of them) that show a drop in milk supply when a progestin-only ("mini") Pill is introduced before 8 weeks postpartum. I don't have the exact reference at my fingertips. I've seen this in clients, and sometimes even see it much later in lactation. The most dramatic case was a mom back at work, pumping for her baby for 4-5 months, who noticed a drop of over 50% within a week of when she began the mini-pill. Since she was pumping quite a lot of milk, the drop was (1) dramatic and (2) measurable. She quit the pill and supply bounced back in about 2 weeks. 

Linda Smith, Dayton OH
Bright Future Lactation Resource Centre
http://www.bflrc.com/bflrc.htm