Probably the best all around headset for Xbox 360 gamers that get online, yet don't want to disturb the neighbors/roommates/kids/wife...

Grab these Tritton True 5.1 Audio Xtreme Headset Dolby Digital Certified headset for $109.84 with free shpping over at Amazon right now.  Joystiq just did a write up on Call of Duty 4 using these and came away impressed.  The Amazon reviews also have great things to say about these headsets including:

I use these for my Xbox 360 and recommend them to anyone who wants to enjoy games at full volume without bothering others (in my case, my wife).

I nearly reached for the phone when, in Rainbow 6 during a Live game, the phone rang in one of the office cubes. It was to the right and behind me, where my phone is in the "real world." However, this time the ringing was in the game! I could also hear that someone left a radio on in their cube to the right of me.

Check it out:  Amazon - Triton True 5.1 Headset