Neno Loje, an MVP, recently posted about dealing with HTTP 503 or 403 errors after installing TFS SP1.  Now Brian Harry has written post that should serve as a quick guide to fixing the common problems that occur when installing TFS SP1.  See Problems installing TFS SP1 for a table that shows each issue, its impact, and how to deal with it.  If you are wondering whether you should install the service pack, Brian posted the list of issues fixed here and here.