I had an interesting question from the field that warranted a response.  The question, in the simplest terms, was:

 "If I have two CRM / web servers (with all server roles installed) behind a load balancer, I'll cut one out from the load balancer to install an Update Rollup. As the Update Rollup install will be updating the CRM organizational and configuration databases that the remaining CRM / web server is accessing, will my users still hitting the remaining online CRM server be affected during the Update Rollup installation?"

In short, the answer is no, they will not be affected.  They can continue using the system while servers are being updated. Database updates are backward compatible so that users hitting the not-yet-upgraded web server after the database is updated will be fine.

All dbUpdates are designed to be forward/backward compatible because in multi-server environments, as is commonly the case with our Enterprise customers, you cannot guarantee all serviced components are at the same patch level.  In fact, practically speaking, this is impossible.  This forward/backward compatibility pertains to not only database updates, but any change that affects more than one "Serviced Component". "Serviced Components" consist of:

All installers (MSIs):

• Microsoft Dynamics CRM Server 2011
• Microsoft Dynamics CRM 2011 for Microsoft Office Outlook
• Microsoft Dynamics CRM 2011 Language Pack (aka Multilanguage User Interface, MUI)
• Microsoft Dynamics CRM 2011 E-mail Router
• Microsoft Dynamics CRM 2011 SSRS Data Connector
• Microsoft Dynamics CRM 2011 Report Authoring Extension (Microsoft Dynamics CRM 2011 BIDS (Microsoft SQL Server 2008 or 2008 R2 Business Intelligence Development Studio) Fetch Extension)

All Databases:

• Organizational
• Config_Db
• Microsoft Dynamics CRM Client for Microsoft Office Outlook

Miscellaneous components:

• Metadata
• Shared Metadata
• Localized Labels
• CRM SDK (Software Development Kit)
• Microsoft Dynamics CRM Solutions
• Reports
• Web.Config

This forward/backward compatibility applies whether the Update Rollup is un-installable or not un-installable. 

Greg Nichols
CRM Premier Field Engineering
Microsoft Corporation