Something not mentioned in the release notes of Cumulative Update (CU6) is that there is a dependency on running the new OCS2009-DBUpgrade.msi before any server updates. If you try to run the ServerUpdateInstaller.exe and apply the server updates without first running the database package you may see an error like this:
Event ID: 30968
Source: Live Communications User Services
Details: The component Live Communications User Services reported a critical error: code C3EE78F8 (Enterprise Edition Server successfully registered with the back-end, but a stored procedure version mismatch was detected. The service will not start until this problem is resolved. Cause: The database schema and the Enterprise Edition Server were updated by different installation packages. Resolution: Ensure both the Enterprise Edition Server and back-end were installed or modified by the same installation package. The service has to stop.
Obviously the error verbiage is a bit outdated with references to LCS, but the error is correct - there is a mismatch between the stored procedure versions which makes the Front-End service to fail to start.
To avoid the issue be sure to apply the latest OCS2009-DBUpgrade.msi package before updating any Front-End servers.