It seems that Janet Elise Rosenbaum of Johns Hopkins University did a study entitled, "Patient Teenagers? A Comparison of the Sexual Behavior of Virginity Pledgers and Matched Nonpledgers," which was published in the January 1st edition of Pediatrics..
The methodology was to compare teenagers who took virginity pledges with teenagers who were similar in all other respects (religiosity, family life, attitudes towards sex, etc.) but did not take the pledges.
SUBJECTS AND METHODS. The subjects for this study were National Longitudinal Study of Adolescent Health respondents, a nationally representative sample of middle and high school students who, when surveyed in 1995, had never had sex or taken a virginity pledge and who were >15 years of age (n = 3440). Adolescents who reported taking a virginity pledge on the 1996 survey (n = 289) were matched with nonpledgers (n = 645) by using exact and nearest-neighbor matching within propensity score calipers on factors including prepledge religiosity and attitudes toward sex and birth control. Pledgers and matched nonpledgers were compared 5 years after the pledge on self-reported sexual behaviors and positive test results for Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis, and safe sex outside of marriage by use of birth control and condoms in the past year and at last sex.Watch the slight of hand here: Previous studies had showed that people who take virginity pledges delay sex longer and have fewer partners, statistically, than those who don't. However those who take virginity pledges are generally much more religious, come from stabler and more religious families, and have more negative attitudes towards premarital sex and birth control than the general population. So this study takes those who have taken virginity pledges and compares their outcomes to people with the same levels of religiosity, family support, and sexual attitudes -- and (surprise, surprise) finds that people who take virginity pledges and those who share their beliefs and lifestyles but haven't taken pledges have very similar outcomes. The piece of paper itself does not prevent premarital sex! Who would have thought?
RESULTS. Five years after the pledge, 82% of pledgers denied having ever pledged. Pledgers and matched nonpledgers did not differ in premarital sex, sexually transmitted diseases, and anal and oral sex variables. Pledgers had 0.1 fewer past-year partners but did not differ in lifetime sexual partners and age of first sex. Fewer pledgers than matched nonpledgers used birth control and condoms in the past year and birth control at last sex.
CONCLUSIONS. The sexual behavior of virginity pledgers does not differ from that of closely matched nonpledgers, and pledgers are less likely to protect themselves from pregnancy and disease before marriage. Virginity pledges may not affect sexual behavior but may decrease the likelihood of taking precautions during sex. Clinicians should provide birth control information to all adolescents, especially virginity pledgers.
Now the study author has a point that whereas the number of virginity pledges signed is often taken as a measure of success for abstinence-based programs, it could well be that these programs are simply making more measurable a segment of the population who are already showing a higher than average propensity to delay sex till marriage. Of those in the study who had taken virginity pledges or who had similar characteristics to those who had, roughly 55% had had pre-marital sex by around age 25. By comparison, a 2002 survey found that 75% of all Americans have had premarital sex by age 20. (There's some fuzziness here in that from the working I think the data in the Pediatrics study may only be the pre-marital sex rate for those who are unmarried -- but I would assume that the pre-marital sex rate for those who are married would not necessarily be higher than for those unmarried so I'm assuming that the statistic holds.)
The real question one should ask, if trying to judge the worth of abstinence based sex ed programs in schools, is whether these programs create a larger demographic of young people who have the characteristics of the pledge takers -- or at least reduce defection rates among those who already have those characteristics -- not whether the piece of paper itself is instrumental in keeping people from having sex. This study does nothing to show whether abstinence based programs increase or decrease the number of students with the cultural characteristics that would assist in saving sex for marriage. Indeed, as constituted, the study doesn't really produce any interesting or useful results at all.
My guess, if someone were to ask, is that public school abstinence programs do not actually achieve much of anything (indeed, the evidence is that no kind of sex ed works) because the principles of cultural and religious impartiality at play in our modern public school system preclude giving students much support in developing a worldview in which saving sex for marriage would seem a worthwhile endeavour. However, one would have to conduct a different study to prove that.