The ball was a full yard beyond the line, but the goal was not given. Had it been at the other end of Old Trafford, for Manchester United instead of against them, I believe the ref would have called it a goal. Along with millions of other conspiracy theorists.
"Disgrace," "remarkable oversight," "injustice;" those are the words used in this piece in the Times. Although the announcers on FSW did not travel down conspiracy theory lane, their commentary was sharp and brilliant: there is no excuse for the game not to get these decisions right.
Even Ferguson (United's boss) stated that such an error could have been fixed in less time than it takes for a goal kick. Cameras are cheap. Perhaps next season?
Update: Here's a picture, and here's a video clip of the incident.