It depends on the context. An "act of war" is defined in the U.S. Code as any act that occurs during declared war or during armed conflict between two countries (although President Bush did call the Sept. 11 attacks an "act of war"). So, technically, if a cyber-attack occurs during a war, it's an act of war; if not, it's not. Whether or not a cyber-attack is grounds for war depends on the nature of hackers' intentions: to spy, by stealing secrets, or to disrupt national infrastructure. Most governments consider espionage—the collecting of information about another country—a crime but not a casus belli. But sabotage—say, pulling down a power grid that serves hundreds of cities—could be construed as one.