I am using Knockout and the Knockout Mapping plugin.
I guess Response is reserved, when I change "Response" to "resp", everything went fine. See http://jsfiddle.net/BBzVm/
Should't you use ko.mapping.updateFromJSON on your success event? Chapter Working with JSON strings on Knockout Mapping site says:
If your Ajax call returns a JSON string (and does not deserialize it into a JavaScript object), then you can use the functions ko.mapping.fromJSON and ko.mapping.updateFromJSON to create and update your view model instead.