In today's world this is a very legitimate concern. The 'Net is a very dangerous place. As parents (and grandparents like me) we need to teach our children about those dangers in the same way we teach them to look both ways before crossing a street and not to talk to strangers.
We can "lock down" their PC's and iPods, but IMHO, TRUST and RESPECT are more potent deterrents to seeking out "inappropriate content." Another thing to do is spend some time with your child and their new "toy". Search for educational apps like NASA, APOD (Astronomy Picture Of the Day) and so forth. I got my 11 year old grandson "hooked" on astronomy that way...
With that said, you need to establish rules of behavior that include definate consequences, and when those rules are broken, do not hesitate to enforce them.
The fact of the matter is, our children will test us because they want to know what their boundaries are, and they want to know whether or not we love them enough to enforce the rules. If more parents did that the world would be a much better and safer place.....
Just my $.02....