The week of May 7, 2007, an Xbox 360 update will enable the Xbox 360 with Windows Live Messenger features. This includes text chat with up to six people, all while playing games or enjoying other content.
Your LiveID will enable you to see other people on your contact list, allowing you to see their online state and reach out to them.
Microsoft Press Pass: "Instant Messaging Comes to Xbox 360, Expanding the Largest Social Network on Television".