function addComputedProperty(model, computedProperty, dependencies, fn){
  var callFn = _.debounce(function(){
    var args = dependencies.map(function (property){
      return model.get(property);
    });
    model.set(computedProperty, fn.apply(null, args));
  });
  dependencies.forEach(function(property){
    model.on('change:' + property, callFn);
  });
}

// Example usage:
var model = new Backbone.Model({
  firstName: "Joe",
  lastName: "Schmoe"
});

addComputedProperty(model, 'fullName', 
                    ['firstName', 'lastName'],
                    function (firstName, lastName){
  return firstName + ' ' + lastName;
});