Halo 3's launch was heard by jedi throughout the galaxy – it was as if a million voices cried out in unison, then suddenly silenced. Oh, and Xbox Live also was affected a bit:
Microsoft has told Next Generation that the sheer amount of gamers attempting to play Halo 3 via Xbox Live resulted in some "minor issues" with the online service. The launch of the title, combined with problems playing it online, also led to vastly increased traffic on Xbox.com which suffered operational hitches too.Microsoft labeled Halo 3's launch day as the "most active Xbox Live gaming day in history" after more than a million people had been logged playing Halo 3 online.
A Microsoft spokesperson confirmed, rather vaguely, that there had been some issues, all of which have now been resolved.
via Games Radar