In one of Ina May's books (can't remember which one) she says that, in her experience, women who have regular sex throughout pregnancy are *less likely* to go postdates than women who don't but she makes it clear that this is just an observation of hers not any type of study.
I personally agree with some of the PPs, it works if you're ready to go into labour anyway and doesn't if you're not.
Physiologically I would think that sex which combines nipple stimulation, application of prostaglandins and orgasm (for uterine contraction) would be the most likely to be effective.