Well, yes, it continues a bit more...
Yesterday I explained WebHttpBinding which is dedicated to REST world. It is however just one aspect of the new REST support in .NET 3.5. Here is another one - you can invoke remote WCF services from ASP.NET AJAX.
To resolve this, there is "WebScriptServiceHostFactory" which adds support for those js proxies for each service endpoint. It internally uses WebScripeEnablingBehavior which is derived from WebHttpBehavior. When WebScriptEnablingBehavior is attached to a service endpoint, it adds a special request "dispatcher" (precisely, EndpointDispatcher) to the host to handle requests to ./js or ./jsdebug as to return js proxy string.
I don't explain the mechanism a lot here, but in case you are curious, ChannelDispatcher, EndpointDispatcher and DispatchRuntime have a lot of customizible things at service host.
(In my previous post, I didn't explain much about WebHttpBehavior, an implementation of IEndpointBehavior. It actually plays the primary role on the REST binding support such as, setting "message formatter" to serialize request parameters using JSON serializer, to bind to and generate parameters from "URI template", etc. etc.)
The entire service host setup above is valid to standalone ServiceHost too. But it does not make sense to do so - the primary purpose of those js proxies is reference from ASP.NET AJAX web pages. Hence there is almost no use for standalone hosting.