You have to wonder what sort of system allows this to happen, but still. It seems that ISPs in Pakistan tried to censor YouTube ("because of content deemed offensive to Islam." says the BBC) by redirecting users to a different address, and they somehow propagated the blocking to ISPs in other parts of the world.
The BBC report quotes Google:
"Traffic to YouTube was routed according to erroneous internet protocols, and many users around the world could not access our site," it said. "We have determined that the source of these events was a network in Pakistan. We are investigating and working with others in the internet community to prevent this from happening again."
The Guardian has the AP report.
