Bungie does not own Halo anymore. Correct me if I'm wrong but I believe 343 owns it now (if they are the wrong details, sorry).
You know, everyone is going to Reach now and I must admit, Halo 3 has a reputation for the campaign and mostly the forge maps Foundry and Sandbox. Now, Reach has a reputation of the Matchmaking/multiplayer and the forge options. So, Halo 3 is losing a few players a day. It sounds sad but if you think about it, there is still MANY, MANY Halo 3 players.
I believe there will be more people on Halo 3 on September 25th. Most people know it is the anniversary. I may be wrong about all the people but thats my theory. I know many people can't wait for the day (including me). So, maybe they will make an anniversary playlist. I'm not sure what they will do, but I do believe they will do something.