Similar attacks occurred recently with some U.S. government sites including the state department which they though may have come from North Korea.
With all the events that have happened in Iran and how twitter was being used there by protesters I would not be surprised if this attack was coming from within Iran. (So much so that Twitter changed their maintenance time so protesters could use the service) In addition, Ahmadinejad was recently sworn in recently which might suggest a consolidation of power within the bickering government there.
If that is the case it would present an interesting situation. There was a recent editorial in the Christian Science Monitor addressing how the U.S. government should or should not react to the current situation in Iran. We’ll see what happens.
Now all the internet nerds won’t know what to do. More updates to come.