some firsthand experience:
First off, a few minor corrections. What you call the "Virginity Pledge Movement" was actually dubbed "the True Love Waits" movement by the LifeWay Christian Resources Management Team back in 1993. They're a part of the Southern Baptist Convention (in fact the first 100,000 cards were delivered to the '93 SBC in Houston, Tx). Nowadays, many more churches than the SBC practice this. In fact, I know people from the UMC and a few nondenoms who have signed the cards.
The church I grew up in was Shady Oaks Baptist of Hurst Texas (yep, SBC). They participated in the program from the outset, and I got to see it's effect on up until I graduated in 1998. Each year from '93 to '98, the Youth Minister would, at the start of each school year, put True Love Waits posters up, and have a special service where we would sign the cards, and send them into the SBC. Each year, everyone in the group would sign them, as ApoxyButt said earlier, mostly as a peer pressure thing. Quoth the card:
"Believing that true love waits, I make a commitment to God, myself, my family, my friends, my future mate, and my future children to be sexually abstinent from this day until the day I enter a biblical marriage relationship."
Each year, they'd give us these little silver rings to wear on our wedding ring finger, and each year I'd either misplace it or put it on my keychain (I looked older than most, and people kept thinking I was married...) A few weeks later, nobody thought about it much again...
I never really had a problem with keeping my virginity (throughout Junior High and High School I was one of those guys who would sit in the back of the classroom and program his calculator all day. I was really quiet, didn't get out much.) In fact, to this day I've still not been on a date, and haven't even kissed anyone, but that's cheese for another node 1... The thing that was interesting to watch was exactly how little the whole practice meant to my peers. (it's interesting to note back in 1997 there were actually 3 guys who stuck together and refused to sign the cards. They were honest with themselves, and didn't want to make a promise they wouldn't keep, and most of us (then) youth respected them for it. Most adults had a very different reaction.)
Fast forward 5 years to graduation. Out of the 10 people who took "the pledge" each year with me, 5 had broken it (and 3 had children to show for it). Of course by then, we had a new Youth minister who tried "dynamic and new" approaches to keeping the kids virgins, with similar results... As of 2 months ago, when my mom and stepdad took over the college department at that church, the class was made up of about 20 or so people, with about 4 or 5 single mothers who came through the same program. (Interestingly enough, after I looked into it, Texas has the highest teen pregnancy rate in America, and Tarrant County (where I live) has the highest in Texas. Must be something in the water.
I don't know many folks outside of Texas who have participated in the program, so I couldn't tell you how it turned out elsewhere, but here it was pretty much a joke.