Summary - Previously managed MOM Agents start appearing in the Pending Actions node of the Administrator MMC. You are unable to discover new agents.
If the Management Server also has the MOM Database role and was upgraded to SQL Server 2005, the .NET Framework 2.0 would have been installed. There is an issue with MOM 2005 SP1 discovery and the .NET Framework 2.0 which requires a hotfix.
If SQL Server 2005 has been installed also see http://support.microsoft.com/?id=917615 for a list of hotfixes which need to be applied for SQL Server 2005 support.