Use this tool to enforce comments being added by developers when the check-in their code to a tfs source repository
http://blogs.vertigosoftware.com/teamsystem/archive/2006/08/22/3417.aspx
More information can be found here about how to create check-in policies yourself...
http://blogs.vertigosoftware.com/teamsystem/archive/2006/02/27/2302.aspx
http://blogs.msdn.com/jmanning/archive/2006/01/21/515858.aspx