Update: As pointed out by various members of the community, the reports of this story appear to be based on a hoax. Original story follows.
Anonymous hacking group, Anonymous, has taken the responsibility for the Xbox Live issues users have encountered today. Microsoft is currently working to fix the situation. Meanwhile, some people who have managed to sign in are having trouble with other matters, like this one guy whose entire friends list has been deleted. Microsoft has issued the following update:
"We are aware of an issue members are having sending and receiving messages, seeing friend’s statuses, sending or receiving game and party invites, and accepting friend requests on Xbox LIVE. We are currently working as fast as we can to get this issue fixed, thank you for your patience during this time and we will update you with any new information we receive."