Wow people in this thread have been watching way too many movies.
Number 1: It wasn't a "hack". These were Denial of Service attacks. What a DoS means is that someone using a bot network had their network request data from the web servers the web sites run off of. Enough zombie comps in the bot network sending thousands of requests a second and boom you get Denial of Service.
In other words they flooded the web servers with useless requests so that the servers either crashed or were severely slowed down. For example it happens when small websites get exposured and are bombared with so many visits the budget servers they are run on can't handle them so they crash.
Number 2: What was affected in this denial of service attack was the public government run websites; for example Whitehouse.gov, blah blah blah. Private web mail servers used by the government weren't affected.
Number 3: Things like power grids and really anything of vital importance to the country are run off of private Intranets which are usually restriced to only that area/region/etc and are not connected to the regular Internet in any way shape or form.
So yeah calm down people.
Edit: As far as who actually did it, they're are plenty of bot networks around. Many in china, russia, the u.s...any country with computers actually. And plenty of people who control these bot networks frequently rent them out to anyone. It's their business basically. They can shutdown a companies website, crap like that for fees. Hell they even attack competitors bot networks...god I read to much crap lol.