My kids and all of their friends have access to several channels of TV designed just for them, broadcast 24/7. They have vast libraries of video games at their disposal. They have iPods with tens of thousands of songs to listen to at all times.
What's the effect of all of this? Hmmm. I think it's something to be researched and blogged.
My bet is that the stimulation itself is not nearly so bad as the replacement of other activites with idleness. My son recently asked for stories from my childhood the other day. My buddies and I were pyromaniacs. We learned how to make gunpowder, made our own rockets, experimented with all manner of chemistry and plenty of other things. In the process we learned how to research things, patience in the development of our devices and a bit of planning.
Other kids in our neighborhood were into making their own go karts. They learned all kinds of mechanical skills. Where is that now? When was the last time you saw a bunch of boys taking an old lawnmower apart to get at the engine?
Update: It occurred to me today that this might be the unspoken reason that kids are being overscheduled these days. Organized activities are one of only two reliable antidotes to te constant availability of video stiumlation. The other is enforcement of draconian limitations on access to these things. That leads to battles with the kids. It's easier just to overschedule them and keep them busy.