Uninstalling .NET 4 is not always an option, for example if you have installed the Team Foundation Server 2010 Extensions for SharePoint.

This problem can also be fixed by modifying .config files. See the workaround for Known issue 1 in http://support.microsoft.com/kb/2536599

Starting the FIMSynchronizationService still failed after modifying the .config file, but when I started it from Central Admin as mentioned in the Workaround for Known issue 3, it started!