Xbox live is down. No games that I know of so far can play online.
Best bet is to not connect to LIVE at all. If your on Gears of War, Even going to Campaign(While signed in to live), won't work. I beleive if you disconnect the internet it should work. Apperantly the workers at Xbox live are looking into it(So they said at the Xbox.com forums).
As of now, All i can do is wait...Bored to death..
I doubt its nature related..Probably updates..I'm looking into it...
UPDATE: There may be a connectivity with this earthquake
The article says that it has effected some parts of U.S. If this part is where the Xbox servers are located..Then...That may be one reason..Not sure yet..Just a thought..The article also says that communications may take 2-3 weeks to fix..Hope Xboxlive is not down that long
Xbox live works again!