Update (As of 3am ET)
It seems the normal Xbox Live Services are now back to normal. If you are still having issues, the Xbox Support team recommends logging out and power cycling your Xbox system.
At the moment the only issues with Xbox services seem to be on the Xbox 360 side, with issues with the Call of Duty: Black Ops game and the MYTF1 app.
- (Original Post)
It seems that Microsoft is having with its Xbox Live Services. As of this post, Microsoft is reporting issues with accessing its Xbox Live Core Services, as well as Purchase and Content Usage, and the Xbox 360 MYTF1 app.
According to Xbox Support, Microsoft has teams currently investigating.