A 5-year study of 150,000 kids in 5th through 8th grades in North Carolina found modest drops in test scores for kids, especially poor kids, who got a home computer during that period. The time they spend on the computer comes out of reading, TV, and homework time. The study ended in 2005, so it missed the recent Facebook* phenomenon, where the computer takes the place of everything.
I guess what it shows is parents have to pay attention to how much time a kid is spending on the computer. No biggie.
*I wouldn't have capitalized that, figuring that the word has achieved a genericness, like Kleenex, that allows it not to be capitalized, but this stupid program kept putting an annoying red line under it unless I capitalized it. Worse, it did the same to Kleenex. Don't the Google programmers know Kleenex is generic now? How about thermos? Yep, understands that. And aspirin.