The Associated Press has an article on the use of age verification for social networking sites. Several government officials are pushing for various age verification techniques which include driver's licenses or credit cards.
I think MySpace would suffer quite a bit if they were to require something like credit card verification. I, for one, would never give MySpace my credit card number. With as many problems as it has (all those annoying errors!), I wouldn't trust it to keep my credit card number secure. Whether that's a justified fear or not, I still have it and I'm willing to bet others feel the same.