Did you ever take note of the subject of children's and teenager's conversations recently? If you listen in there is probably a 95% chance that they will be discussing a cartoon or television program, the newest toy or gadget they want, their favorite musician/celebrity, or their body image. You will rarely hear anything personal, creative, cultural, or inventive. If this is somewhat like your children, don't pass by the red flag.
If you look at page 10 of this PDF at the chart illustrating scientific studies vs. news reports of the Effect of Media Violence on Aggression, it is clear the media wants us to think in its favor. The role of media in childhood obesity is also very clear and does not leave any room for doubt. I highly recommend reading the latest systematic review published in November 2008 on Media and Child/Adolescent Health.
If you are looking for a good parenting book to read that addresses this issue, I recommend Parenting Well in a Media Age by Gloria DeGaetano, it was a very easy, educational, and enjoyable book to devour.