I volunteer every week in both my third and first grader's classes. It's a public school; our G/T program doesn't start until 4th grade.
Some of the kids read below grade level in the class. Inevitably, the teacher has me work with those kids. In chatting with these kids, it seems like they watch ALOT of TV and have unfettered access to their Wii, XBOX, whatever. I mean, they have a TV in their room, and some watch it until midnight when they are "supposed to be asleep."
Are they below grade level b/c of their heavy video/computer use? Or is it b/c English isn't their first language, or their parents aren't educated, or... a million other reasons.
Obviously, I don't know. It could be an epiphenomenon, but I suspect that all of their TV/video usage isn't helpful.