Over the next several months Microsoft will deploy the Exchange Online December 2011 Service Update. As a part of this update, we will also make high availability architecture enhancements in all datacenters that host Office 365 tenant domains. These updates will be staggered globally, beginning in December 2011, and we expect full deployment to complete within six months. Approximately one week before the service update is deployed to your region we will post more information in the Planned Maintenance section in your Service Health Dashboard portal. Please check this resource for awareness of when your update is scheduled to begin.

Read the complete blog at http://blogs.technet.com/b/exchange/archive/2011/12/07/exchange-online-december-2011-service-update.aspx

Read my favorites blogs:

Designing a backup less Exchange 2010 Architecture

Step by step guide for upgrading Active Directory from Microsoft Windows 2003 to Microsoft Windows Server 2008

Microsoft Exchange 2010 CAS Array – Steps and Recommendations

Appear Offline in Microsoft Office Communicator Server 2007

Microsoft Exchange 2010 Test cases

Microsoft Exchange Server 2010 Disaster Recovery