Here's a nice blog series I ran across on requirements management with Team System, Team Foundation Server and some 3rd party products.  We are also working on a whitepaper to provide an overview of this.

http://blogs.msdn.com/slange/archive/2007/11/06/requirements-management-in-tfs-part-1-of-4-overview.aspx

http://blogs.msdn.com/slange/archive/2007/11/06/requirements-management-in-tfs-part-2-of-4-tfs-out-of-the-box.aspx

http://blogs.msdn.com/slange/archive/2007/11/06/requirements-management-in-tfs-part-3-of-4-integrations.aspx

http://blogs.msdn.com/slange/archive/2007/11/06/requirements-management-in-tfs-part-4-of-4-summary.aspx

Brian