Microsoft Knowledge Base Article 914034 contains the following introduction:
When you back up a domain controller that is running Microsoft Windows Server 2003 Service Pack 1 (SP1), a new event error message is logged for each writable domain or application partition that the domain controller hosts. This is true if the partition is not backed up in a given time period. The time period is called a backup latency interval. You can set a registry value to specify this interval in days.