Yes, my 15YO has one, as do I. Yes, I have her password, and she knows I check her account almost daily. Yes, I also check her email. Yes, her account is set to private, and yes, I know all the people personally she has on her friends list.
This boils down to parental responsibility. If your child is going to be on MySpace, or any other service, you have to set the rules and stick to them. You must check up on them. You MUST BE THE PARENT, NOT THEIR FRIEND. MySpace is not bad, nor is IM, Xanga, Facebook, etc. If you think keeping your child from MySpace is going to keep them innocent, think again.