A common misconcept is Json is required for Ajax. Ajax can work with any format, but Json has significant advantage over XML. When making an Ajax call to a REST service, the request header can be specified with an "Accept:" format, but it is just a "wish", and will be granted only when the service has a capability for the requested format. So the following code will return error when the service doesn't response with Json data:
var data = response.get_responseData();
data = eval("(" + data + ")")
Also keep in mind, Visual Studio (2010 RC) doesn't provide java script intellisense for REST response data. In contrast, Ajax-enabled WCF service can emit a script proxy (via asp:serviceRefernce), so it has full intellisense support, and has built-in serialization/deserialization support with default Json format.