Xbox Live, Xbox.com and the forums will be offline for maintenance between 2am and 4pm Pacific time.
Update 06/16/09:
Xbox Live Down for 24h scheduled Maintenance.

Update 04/21/08
Xbox Live down for scheduled Maintenance

Update: 01/04/08:
Microsoft gets sued for Xbox Live Issues, Service still has Problems

Update 12/31/07:
Xbox Live still having Problems

Update 12/23/07:
Xbox Live issues confirmed, MS works on resoliving connectivity issues.

Update Nov. 13th
Xbox Live down for Maintenance at 1am
Update Oct. 17th
Xbox Live Down Again for Maintenance

On xbox.com you can currently read in several languages:
"Xbox.com and the Xbox Live® service are currently down for maintenance. We’ll be back soon with amazing new My Xbox® features!"

14 hours of downtime? I cannot help it, but this a very long downtime for a paid service. Looks like Microsoft does not having an appropriate staging environment and no effective way to upgrade Xbox live - Does Bill know about this?

There are no major new features announced, so nothing to look forward too. See the maintenance announcement on Major Nelson's Blog.
He points out that this is not a Xbox 360 dashboard update.