Yesterday I answered a question on SO
What I created is basically a simple example that sends a string to a classic asmx webservice which in turn answers again with a string, which is then displayed on the client-side. A simple greeting service basically :) .
The second example is more complex (although ASP.net makes it really easy to handle) in the sense that I'm sending back a whole object graph: a Person object containing another Location object. For programming such interactions I strongly recommend to use Firebug
I not going into too much detail now since I don't have the time and moreover I think the example speaks for itself. I just want to mention some important parts regarding the WCF service.
[ServiceContract(Namespace = "AjaxWebservices.Services")]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class PersonService
You invoke it with the namespace of the service. But having a WCF service, the namespace definition defined in the ServiceContract (see above) will be the one that actually holds and not the class namespace. In this example above I configured them to have the same name, just for simplicity.
For the rest take a look at the example.