Looking through the event logs, post Forefront installation, a particular error keeps cropping up, stating Event ID: 6014 Source: GetEngineFiles on the ScanEngine called CAIris.

Microsoft Forefront Server Security encountered an error while performing a scan engine update.
   Scan Engine: CAIris
   Update Path: http://forefrontdl.microsoft.com/server/scanengineupdate/x86/CAIris
   Proxy Settings: Disabled
   Error Code: 0×80004005
   Description: An error occurred while checking if an update was available.

The Forefront Administrator console shows the last update status of the engine as Unknown and the ProgramLog.txt file located in the installationdata folder showed this message

Fri Oct 17 14:40:00 2008 ( 8604- 8420), "ERROR: Unable to load manifest from: http://forefrontdl.microsoft.com/server/scanengineupdate/x86/CAIris/Package/manifest.cab : WinHttpClient send request returned an invalid return code 404."

The error only appears on the CAIris Scan Engine and not on the other four Engines activate – the simplest solution I could find so far was to disable the CA Scan Engine and enable one of the others.

Update: I finally found the reason on the Microsoft Forefront Server Security Blog which essentially explains that CA InoculateIT has been consolidated into the CA Vet option.

More information can be obtained from this Microsoft Article: Key points and frequently asked questions about the Computer Associates antivirus engine consolidation