You need to add a value attribute for the radio-button to map to. Also the value won't be a boolean, but a string after this. I use the following extension to use boolean values for radio buttons. (live: http://jsbin.com/exokav/8):
ko.bindingHandlers.checkedAsBool = {
init: function (element, valueAccessor, allBindingsAccessor) {
var observable = valueAccessor(),
interceptor = ko.computed({
read: function () {
var val = ko.utils.unwrapObservable(observable);
return ((val !== null) && (typeof val !== "undefined") ? val.toString() : val);
},
write: function (newValue) {
observable(newValue === "true");
},
owner: this
});
ko.applyBindingsToNode(element, { checked: interceptor });
}
};
Based on this answer by RP Niemeyer.