my last "simplification" of the the if-then-else logic in BaseHttpServer placed the return in

the "if (request.UserAgent != null)" branch in the wrong place: as a result BaseHttpServer would not do 
anything if the user agent header field was present....ARGH! BAAAAAAD.

OpenSim/Framework/Servers/BaseHttpServer.cs

@@ -161,8 +161,8 @@ namespace OpenSim.Framework.Servers
                     {
                         m_log.DebugFormat("[HTTP-AGENT] Handler located for {0}", request.UserAgent);
                         HandleAgentRequest(agentHandler, request, response);
+                        return;
                     }
-                    return;
                 }
 
                 IRequestHandler requestHandler;