Yesterday, according to Bungie, the 500,000,001st game of Halo 2 was played on Xbox Live. To be clear, this number does not include player-sessions or games that were lost because Bungie.net was down, or other technical issues.
While it's certainly an achievement Bungie should be proud of, it's hardly a surprise. 6.4 million units of the game were sold in less than three months. Even if another copy was never sold, and even if only 5% of those 6.4 million units were associated with an Xbox Live account (that's 320,000 unique players) -- and given that the game has been in release for 573 days -- each of those 320,000 players would only need to average (roughly) 3 matchmaking games per day. C'mon, the Joystiq crew clocks those numbers before breakfast.
Let us know when you hit 1 billion, Bungie. Then we'll be impressed. (We kid, we kid.)