Okay that was a smoother installation than I expected. So Ari recommended using this from the command prompt
Exchange2007-KB949870-x64-EN.msp /quiet /lxv* KB949870.log
I Did bit found it a bit scarey that you get no feedback on what is happening so I tried /passive instead.
But what I did find was this in the Application event log when the rollup was applied:
Event Type: Information
Event Source: MsiInstaller
Event Category: None
Event ID: 1022
Product: Microsoft Exchange Server – Update ‘Update Rollup 3 for Exchange Server 2007 Service Pack 1 (KB949870) 8.1.291.2’ installed successfully.
Another check is to use Add/Remove programs, check the “show updates” and then under “Microsoft Exchange Server 2007”, you should see “Update Rollup 3 for Exchange 2007 Service Pack 1(KB949870)“
Essentially you can query the subkeys for this: SOFTWAREMicrosoftWindowsCurrentVersionInstallerUserDataS-1-5-18Products461C2B4266EDEF444B864AD6D9E5B613Patches