You might remember some issues with Xbox Live just before Christmas that were fixed soon after, well it appears they are back and Microsoft have responded. They updated their support page, which can be found here, with the following message:

Users may experience intermittent issues with tournaments, posting statistics, matchmaking, and sending messages. We are aware of the issue and are actively working towards a resolution. We apologize for any inconvenience.

Maybe Microsoft sold a few too many Xbox 360's at Christmas to handle the new influx of Xbox Live users :).