See: Schneier on Security: Titan Rain, and Time
There seems to be a well-organized Chinese military hacking effort against the U.S. military. The U.S. code name for the effort is “Titan Rain.”
I can tell you that if a foreign military tried to break into a government/military installation it would be viewed as an act of war. Is hacking really any different? With penalities for hacking becoming more and more severe how long will it take before an organized effort lead by a government is viewed an act of war?