The GenCheckinNotesUpdateWorkItems task is defined in the Microsoft.TeamFoundation.Build.Tasks.VersionControl.dll. The Microsoft.TeamFoundation.Build.targets file contains an instance of the GenCheckinNotesUpdateWorkItems task that Team Foundation Build uses as a default implementation for the GetChangeSetsAndUpdateWorkItems target. You can also customize the build process by defining a different instance of the GenCheckinNotesUpdateWorkItems task for a build type in the TfsBuild.proj file. For more information about how to customize a task, see How to: Modify the Scope of Changesets and Work Items Associated With a Build Type.
The Microsoft.TeamFoundation.Build.targets file is stored in the <root>:\Program Files\MSBuild\Microsoft\VisualStudio\v8.0\TeamBuild folder on the Team Foundation Build computer.