It looks like Xbox Live is having a bit of an outage right now and Microsoft is working
hard to get the system back online. I would attribute this to the some 2 million people who picked up
Halo 3 today but we haven't received confirmation of that (someone could have just kicked an outlet out of a wall). The outage seems to be localized to the web side of the service as I was able to play and send messages earlier this evening.
Update: It looks like everything is now back and
running smoothly.