This type of attack was described in detail in an October Northrop Grumman report, (pdf) commissioned by the US-China Economic and Security Review Commission. Analysts concluded that "China is likely using its maturing computer network exploitation capability to support intelligence collection against the U.S. government and industry by conducting a long term, sophisticated computer network exploitation campaign."
At least 10 to 20 terabytes of sensitive data had been taken from U.S. government networks as part of what the report's authors called a "long term, persistent campaign to collect sensitive but unclassified information."
For the past few years, China has been focused on moving its economy to the next level, said James Mulvenon, director of Defense Group Inc.'s Center for Intelligence Research and Analysis. China built its economy processing products for export, but it is not known for cutting-edge research and development. The country has been taking steps to spur innovation within its borders, pressuring multinational companies to build research labs in China and developing the talent to eventually replace these businesses with indigenous competitors.
Mulvenon doesn't find it implausible that a nation such as China would spy on U.S. companies.
"If you're having trouble [innovating] or if you want to prime the pump, the best way is to go out and steal cutting-edge IP," he said. "It's a plausible explanation for why they would go after Silicon Valley companies on such a broad scale because they're really trying to jump start IT innovation in China."
John Ribeiro in Bangalore and Jeremy Kirk in London contributed to this story.