"A computerized air traffic system experienced a temporary outage in South Florida, forcing controllers to ground some flights and resulting in at least four instances in which planes almost came too close together, officials said Tuesday."
"Monday's outage affecting the Miami Center radar complex lasted about an hour, but a backup system was activated and flights in the air were not in danger, said Kathleen Bergen, a Federal Aviation Administration spokeswoman in Atlanta."
This didn't last too long, but it shows the affect just a short outage would have on flights in the US...even in the even of a terrorist attack on the radar sites that would be horrible..