Someone pointed me to a study that's probably the one I was thinking of in my earlier post.
The results were...mixed:
* Students taking a pledge of virginity were indeed more likely to delay sexual activity (by an average of 18 months)
* It was most effective for students between 15 and 17 years old, and had no effect on students over 18
* While the delay until initiation of sexual activity was about 1/3 longer among those that took the pledge...they were 1/3
less likely to have protected sex once they did become sexually active
* Only 12% of the participants remained abstinent until marriage
* The effectiveness of the pledge decreased when a larger percentage of a school's population took the pledge.
link