In Act V of Macbeth, what is the meaning of Macduff's line "no man of woman born ..."?Is this some reference to Macduff being born by, perhaps, some form of C-Section?
Yes, this reference in Act 5 to Macduff's birth is a reference to a Cesarian section. In Act 4, the witches' prophesy states that Macbeth shall not be harmed by one born from a woman. Macbeth narrowly interprets this advice by assuming that by nature, every person is born from a woman. However, he does not consider that the prophesy may mean a "natural" birth through the birth canal. As stated in the previous response, C-sections were not common at the time and were only used as a last resort in the case of emergency. In Act 5, Macduff says that he was "untimely ripped" from his mother's womb, suggesting that he was also born prematurely. It is possible that his mother was ill or had gotten into some type of accident that endangered her life, causing a local practitioner to take the baby from her womb earlier than planned to save his life.
When Macbeth and Macduff have their confrontation on the battlefield in Scene 8 of Act 5, Macbeth tells his opponent: "I bear a charmed life, which must not yield / To one of woman born." Macduff replies: "Despair thy charm, / And let the angel whom thou still hast served / Tell thee Macduff was from his mother's womb / Untimely ripped." Since Macduff is alive, there can be no doubt that he was born, as you suggest, by some form of Cesarean section. Such operations must have been terribly dangerous and painful in those ancient times before they had any kind of anesthesia, but they must have been employed as a last resort when the infant, for whatever reason, could not come out naturally through the birth canal. Macduff's bad news destroys Macbeth's last hope, and the tyrant is soon killed.