I suppose "as rare as hen's teeth" is too vague for your purposes. :-) A big Swiss database study reported a rate of 1 in 10,000 in women with no previous cesarean.

Rageth JC, Juzi C, Grossenbacher H. Delivery after previous cesarean: a risk evaluation. Swiss Working Group of Obstetric and Gynecologic Institutions. Obstet Gynecol 1999;93(3):332-7.

Since the study was done in Europe and published in 1999, misoprostol (Cytotec) would not have been a factor, so doubtless in countries like the U.S. where induction with Cytotec is common, rates would almost certainly be higher.

While I'm at it, let me add that I'm reviewing papers for the VBAC chapter for the new edition of Obstetric Myths Vs. Research Realities. Some of the papers are about factors associated with uterine rupture, not just the effect of prior cesarean surgery. Having a prior D&C has popped up more than once as an associated factor.

-- Henci