Following the downtime of Xbox Live yesterday, some Xbox 360 users are experiencing connection issues right now because Microsoft is posting TrueSkill statistics on Halo 3.
The Xbox support page calls the issues intermittent. Update: Looks like the issue just got more severe as Microsoft updated the support page with: "Users will not be able to connect or log into Xbox LIVE. We are aware of the problem and working to resolve the issue." Update: Xbox LIVE supposedly up again, but still some issues might still occur.

We see quite some hits on our Xbox Live Down stories indicating that currently lots of Xbox users cannot connect to Xbox Live.
Not sure why posting data is requiring to shut down connection to the service.
Via the Xbox Support page.

The Xbox 360 will get an exciting Dashboard update this Fall, that will make the console much more attractive again especially at the new low price points.