Yes, it is.
And, NO, I have not yet read Headley, Fuck, Fuck, and Curti’s bitch paper.
If someone is a member of the British Veterinary Association and would like to send me a PDF of the paper, I promise to read it and report out to the world.
“Ovarian Teratoma in a Bitch,” S.A. Headley, E.J. Fuck, E.T. Fuck, and C.E. Curti, Journal of the American Veterinary Medical Association, vol. 191, no. 1, July 1, 1987, pp. 81-83. and also Veterinary Record, 2006 Apr 22;158(16):565-7
My understanding is that spaying prevents cancer of the uterus and ovaries in the same way that clear cutting prevents forest fires.
Update: Here is the paper, and I have to say it was less interesting than I had hoped as it deals with a single incident. The most alarmingly and eye-opening part of the paper was the fact that the owner went so long before taking in his dog despite the fact that "he had noticed an unusual increase in the size of the dog’s abdomen almost one year previously..."