Hackers in China have recently gotten attention for reportedly attempting to steal industrial technology secrets and hacking US satellites, and now a Wall Street Journal report claims Chinese citizens were discovered hacking US Chamber of Commerce in May of 2010. The first breech is believed to have occurred in November of 2009 or even earlier, giving the hackers at least six months of administrator-level access to the Chamber's network. It isn't know (or wasn't revealed) exactly how much information was stolen; the Chamber says that fewer than 50 of its members were compromised, but they also said the hackers stole at least six weeks of emails from four individuals who worked on Asia policy.
As to how the hackers got in, it sounds like a "spear phishing" attack was to blame, with targeted users clicked on fraudulent links or email attachments. The FBI found out about this breach and alerted the Chamber, which then hired outside investigators to determine the extent of the data theft. The Chamber has since overhauled its network security and prevents its employees from taking mobile devices into countries with a high infiltration risk (like China) — but there's still some strange and suspicious behavior, like a printer spontaneously printing pages of Chinese characters. At this point, the Chamber feels the best thing it can do is find breeches quickly, because it believes it's "nearly impossible" to keep hackers out entirely.