I was fine this afternoon, but kept getting kicked off tonight and at one point couldn't log on. Sure enough, says this on the support page:
Users may experience difficulties when matchmaking and using client voice communication within Xbox LIVE.
Users may not be able to connect or log into Xbox LIVE. We are aware of the problem and working to resolve the issue.
We apologize for any inconvenience this may cause and thank you for your patience.
And yes, the downtime was yesterday. If you visited xbox.com yesterday morning you didn't have access to gamercards.