So upon reading all the responses so far it really seems to be just the opposite of what I was first thinking.
Let me tell you my initial line of reasoning and the thoughts I've had since reading this thread.
So, we know that breast stimulation works as a form of induction. I agree that it generally only works when the baby/mother are ready for labor. My thought was if a woman is getting breast stimulation daily she is essentially giving a small shot at induction daily so she would theoretically go into labor *as soon as* she was ready.
BUT the anecdotal resposes indicate that this is probably not the case SO
I started thinking about how when I induced lactation just a little a few years ago my cycles immediately spaced out to be incredibly far apart and that in tracking my fertility and reading TCOYF I learned that my body was having several "false starts" to ovulation by making some estrogen, but not enough to reach the threshold necessary to actually ovulate.
Maybe there's a similar threshold with labor and nursing women have a higher level of oxytocin to reach for it to be effective. (I wonder how pit inductions go in nursing moms. . .)
THEN I thought a little more and I remembered that, duh, there are other hormones involved in the start of labor. Estrogen levels rise days before labor starts. In the presence of prolactin more estrogen is needed to do it's job--It would be NO WONDER for nursing women to have LONGER gestations than average if anything. Nursing might delay labor in a similar way that it prevents ovulation.
Talk about a 180!
I'd really like to see the effect of nursing on gestation legnth studied.