It most likely wouldn't be considered an act of war, for several reasons. Most cyber-attacks are difficult to trace to a source, and even when they are traced, it's basically impossible to prove (beyond a reasonable doubt) that the attack was state-sponsored. Countries let more serious transgressions slide, because wars are costly and they cause orders of magnitude more damage than any cyberattack.

In the grand scheme of things, economic damage resulting from cyber-espionage or cyber-sabotage is regarded -- or at least should be regarded -- as a cost of doing business on the Internet. Companies should treat DDoS attacks the way banks treat robberies: inevitable and expected. And just like banks, they should simply buy insurance to cover the costs and damages.