Bungie said that they are moving the Halo servers and getting aready for Reach. Bungie said in there post:
This work is *mostly* complete.
Update: This work is complete, however not all of the Halo 3 career stats have finished copying yet. Your stats are not lost and we are not doing any type of stats reset. Your complete Halo 3 career history will return in the next day or so. You may also experience brief periods of stats on Bungie.net returning a message about maintenance mode. Sorry for the inconvenience.
We hope you enjoyed yesterday’s in-game heat wave, but it’s come to a close now that you guys have totally dephlogisticated the server farm. So thoroughly, in fact, that we’re going to retire our dear old Halo 3 servers today and move the game onto the brand spankin’ new Reach-generation servers (well, the ones you got to pound on during the Beta.) During this migration, we’re going to take the file share offline, and new stats won’t show up until we’re done (but will show up eventually.) There aren’t any new features tied to this work (nor will any old ones go away), but just like the way that Halo 2 lived out its retirement years in a quiet corner of the Halo 3 servers, we’re going to give Halo 3 and ODST’s backend features a chance to settle into their new digs on our fresh hardware that’s so powerful it won’t bat an eye at the extra load. The alternative would have us be like a slug leaving a slime trail of old servers in our wake as we push forward into the territory of new games, and while it’s true that most things do go better with slime, we’ve found from experience that servers are that rare exception.
This work will start in the next hour or so, and will continue throughout the afternoon.