I recently had the opportunity to dabble in a concept project utilizing AJAX and have been quite impressed with the implementation of a free library found at http://www.schwarz-interactive.de/ for 1.1 or http://dotnet2.schwarz-interactive.de/ for 2.0.
Implementation is as easy as setting a reference to the library, adding an entry to your web.config and decorating the exposed methods. The decorated methods are exposed to your client side javascript. When those methods are called from Javascript, an HTTP get is performed and the parameters are serialized / deserialized and the original marked up method is invoked. Quite elegant.
In addition the library supports direct support for the majority of the .NET base classes, including the DataSet - as well as exposing an IAJAXObjectConverter interface where you can supply support for serialization of your own custom classes. This gives the ability of the client code to directly access the server side object as a client side object.