Xbox Live had its busiest day ever last Tuesday.  And we’re pretty sure that Modern Warfare 2 coming out on the same day may have had something to do with the milestone.  Internal figures released by Activision indicate that MW2 has become the most successful launch in history across all forms of entertainment.  With an estimated 4.7 million copies sold on the first day in the U.S. and U.K. alone, it’s no surprise, really, that all of those people were anxious to get online and shoot at each other.  That bloodlust resulted in more than 2 million people playing on Xbox Live simultaneously—a landmark Microsoft’s pretty happy about.