Who says Christians aren't trendy? They have their own bandwagons to jump on, just like everyone else. First it was WWJD? bracelets, now it's purity rings. Presumably the next step is for young Christians to have "DANGER! No entry" tattooed on their public mounds.
The theory behind a purity ring is that it symbolises your pledge, to God and yourself, to remain a virgin until marriage. The ring will then not be taken off until it is replaced by a wedding ring.
The problem, as has been pointed out previously, is that most people who make viginity pledges break them. And once they do, they are less likely to engage in safe sex and less likely to seek treatment for STIs. Apocryphal evidence would also suggest that those who are breaking virignity pledges are less likely to do so after mature consideration during a loving relationship, and more likely to get smashed at a party and wake up wondering who the hell that is next to them.
But of course, if there is a way to make money, someone will find it. Purity rings are big business, helped by the fact they are worn by such "celebrities" as Jordin Sparks, Miley Cyrus and the Jonas Brothers. Why is it always teens who wear these rings? Are there any 26 year olds who openly wear them? An awful lot of 18 year olds are still virgins, Christians or not. If there are any proper grown ups out there who are sincere about wearing a purity ring, please prove me wrong, but for Miley Cyrus to wear one is meaningless - chances are at 15 she'd be a virgin anyway. Besides, we all remember the last pop princess who vowed to remain a virgin till marriage...
Maybe the fact that young Christians feel the need to wear these rings says something about their culture. To quote Miss Manners: "Polite society does not recognize such a thing as a chastity ring. It is so polite that it presumes that a lady is chaste unless publicly proven otherwise."