In Halo 3 multiplayer (this was found in the beta, but I'm pretty sure it'll be in the final release), go to the map High Ground. If you go to the defense base where the flag normally is, there's a room with a machine that types out Morse Code. If you know Morse Code, there are lots of random secret messages, such as Frog Blast the Vent Core. A reference to a previous game.

Also, if you stand in that room for long enough, you'll begin to hear very faint music, that sounds like elevator music.

I guess Bungie wanted some of the defense guys to have something to entertain them while they waited for the enemy to try to invade their base!