Update 4pm PDT: Looks like signin is working again (that was quick), Galaga still isn't.
Xbox Live has been having some intermittent issues lately, to go with some issues over the last major holiday. Even Snoop Dogg threatened to leave the platform the last time Microsoft's online gaming service had some issues. Today, Xbox Live is at it again.
These issues do tend to sort themselves out, and so if you're having issues you can check on the status. Until then, we'll try to keep an eye open and let you know when things are back up.