there was a discussion on acts of god on the official forums a while ago.
basically, an act of god is a natural disaster which neither party could have foreseen, predicted, stopped or even prepared for. example, say that the main island of hawai erupts and 90% of the east side of the island drops into sea, creating a tsunami that sweeps away the blizzard building. or presume yellowstone erupts and the entire west coast of America is swept away. both would be acts of god and in neither case, Blizzard would need to repay.
you probably think now: "hey, but that means that if it is not natural cause related, then i can keep blizzard liable for it!". well, no. these are the ToU, specifically the addition:
Originally Posted by Blizzard Entertainment
17.9 Force Majeure. Blizzard shall not be liable for any delay or failure to perform resulting from causes outside the reasonable control of Blizzard, including without limitation any failure to perform hereunder due to unforeseen circumstances or cause beyond Blizzard's control such as acts of God, war, terrorism, riots, embargoes, acts of civil or military authorities, fire, floods, accidents, strikes, or shortages of transportation facilities, fuel, energy, labor or materials.
so, to explain each thing:
acts of god were explained.
war: if america would get a new civil war, you're done for.
terrorism: if a car bomb would detonate outside the server building, resulting in a total destruction of all data.
embargoes: if the government of your country forbids the import of any american game.
riot: if there would be a public uprising and the server building is set on fire.
acts of civil or military authorities: if blizzard would be shut down by the police or the army.
fire, floods, accident, strike: self explanatory.
shortages: well, they can't keep servers running if the power is unavailable for it.