We wanted to let you know that a new version of the Windows Update Agent is being released on Windows Update over the next couple of months. Because WSUS and Windows Update both use the same Windows Update Agent, this means that WSUS-managed end-user who navigates to WU to perform an interactive sync will receive an updated version of WUA as this new agent is rolled out over the next few months. Machines who's end-users don't explicitly navigate to WUA to perform an interactive scan will continue to use the existing version of WUA. This will result in a mixture of WUA versions in most corporate environments. Because WUA is backwards compatible, machines that recieve the newer agent will continue to work just fine with WSUS.
You can read more about this change in the Windows Update blog at http://blogs.technet.com/mu/archive/2008/07/03/upcoming-update-to-windows-update.aspx.
WSUS Program Manager Lead