National Journal's Shane Harris has reliable, knowledgeable sources who believe that the answer is yes:
Computer hackers in China, including those working on behalf of the Chinese government and military, have penetrated deeply into the information systems of U.S. companies and government agencies, stolen proprietary information from American executives in advance of their business meetings in China, and, in a few cases, gained access to electric power plants in the United States, possibly triggering two recent and widespread blackouts in Florida and the Northeast, according to U.S. government officials and computer-security experts.
One prominent expert told National Journal he believes that China’s People’s Liberation Army played a role in the power outages. Tim Bennett, the former president of the Cyber Security Industry Alliance, a leading trade group, said that U.S. intelligence officials have told him that the PLA in 2003 gained access to a network that controlled electric power systems serving the northeastern United States. The intelligence officials said that forensic analysis had confirmed the source, Bennett said. “They said that, with confidence, it had been traced back to the PLA.” These officials believe that the intrusion may have precipitated the largest blackout in North American history, which occurred in August of that year. A 9,300-square-mile area, touching Michigan, Ohio, New York, and parts of Canada, lost power; an estimated 50 million people were affected.
Officially, the blackout was attributed to a variety of factors, none of which involved foreign intervention. Investigators blamed “overgrown trees” that came into contact with strained high-voltage lines near facilities in Ohio owned by FirstEnergy Corp. More than 100 power plants were shut down during the cascading failure. A computer virus, then in wide circulation, disrupted the communications lines that utility companies use to manage the power grid, and this exacerbated the problem. The blackout prompted President Bush to address the nation the day it happened. Power was mostly restored within 24 hours.
There has never been an official U.S. government assertion of Chinese involvement in the outage, but intelligence and other government officials contacted for this story did not explicitly rule out a Chinese role. One security analyst in the private sector with close ties to the intelligence community said that some senior intelligence officials believe that China played a role in the 2003 blackout that is still not fully understood.