This is something of a work-in-progress, so if you know of others, please email me or leave a comment.
From Calling To Courtroom: A Survival Guide for Midwives, an online book. Although some of the state-by-state resources are out of date, it remains the only legal guide of its kind.
Midwives Alliance of North America's list of state laws and chart.
Citizens for Midwifery's State by State Guide to Midwifery in the United States.
CfM's Midwifery and the Constitution
What to Do When Your Midwife Has Been Charged by Katherine Prown, Ph.D.
Suzanne Hope Suarez, Midwifery is Not the Practice of Medicine, 5 Yale JL & Feminism 315 (Spring 1993).
Eric Jansson, In childbirth, the new makes way for the old.
For cases where the courts ruled that midwifery does not constitute the practice of medicine or nursing for purposes of statutes regulating the practice of medicine, see the NARM Position Paper on the Practice of Midwifery.
Legal Aspects of Midwifery from Ronnie Falcao's Gentle Birth Archives.
Midwifery Related Law from Motherstuff.com
The Big Push for Midwives
Jennifer Block, Pushed: The Painful Truth About Childbirth and Modern Maternity Care (Chapters 6 & 7).
Raymond DeVries, Making Midwives Legal: Childbirth, Medicine, and the Law.