Well, as you may have heard, the Japanese ambassador to China died before he was dispatched to China. So instead, Chinese protesters seem to have picked on the ambassador of Japan's overlord (as many in Japan perceive the US to be).
US Ambassador Gary Locke is ethnic Chinese (3rd-generation). Somewhat like the Japanese shops and restaurants destroyed by the mob in various parts of China are owned and operated by Chinese.
From Wall Street Journal (9/18/2012; emphasis is mine):
BEIJING—The U.S. ambassador to China, Gary Locke, was surrounded in his official car by a group of about 50 Chinese protesters outside the U.S. Embassy on Tuesday and had to be protected by Chinese security guards, a State Department spokesperson said.
The protesters caused minor damage to the vehicle, but Mr. Locke was unharmed, the spokesperson said in a statement.
"Embassy officials have registered their concern regarding today's incident with the Chinese Ministry of Foreign Affairs, and urged the Chinese Government to do everything possible to protect American facilities and personnel," the statement said. The ministry couldn't immediately be reached for comment.
The U.S. Embassy in Beijing is close to the Japanese Embassy, where thousands of protesters have been massing over the past several days in a show of anger over the Japanese government's decision to purchase disputed islands in the East China Sea.
Security near the U.S. Embassy was heightened this week, as riot police and others attempted to keep order among protesters. Main roads near the embassy had been closed to vehicle traffic, though they were reopened early Wednesday amid a heavy police presence.
The incident comes as U.S. Defense Secretary Leon Panetta is in Beijing this week meeting with senior Chinese military and civilian leaders.
The statement gave no details about the demonstrators who blocked Mr. Locke's car, and what motivated their action.
Protests in some cities have turned violent, with Japanese cars smashed and Japanese-owned shops and factories forced to close. Nonetheless, protests in Beijing haven't appeared directly targeted at the U.S., or its alliance with Tokyo.
"Chinese security personnel standing in front of the embassy responded and removed the demonstrators from the scene, allowing the Ambassador's vehicle to enter the Embassy compound through another gate," the statement said.
Mr. Locke, the first ethnic Chinese to hold the job of ambassador to China, has become a media celebrity since he arrived in 2011. Many Chinese admire his informal style, and how he and his family mix with ordinary Chinese, in contrast to the aloof behavior of their own leaders.
Mr. Locke was famously photographed by a Chinese traveler carrying a backpack and buying his own cup of coffee at Starbucks at Seattle airport as he flew off to start his assignment in Beijing. The picture went viral on the Chinese Internet, where critics noted a Chinese official would likely be surrounded by personal assistants.