Liu Xiang is injured and will not compete. He has been injured since at least May, when he had his last competition.
In China, an athlete is property of the State and to a lesser extent the People. Perhaps Liu has had enough of being a slave to the system. Perhaps he wants to walk through Chengdu at 3am, sit next to some pretty little girls and have a bowl of jian dan mian.
It worse for athletes that become celebrities, because they don't seek the light like movie stars or such -- the light seeks them because they accomplish the extra-human. I wonder if Phelps will still be able to puff down with his homies now that Bush has given him a call.