SomeController.cs

    public class ClientController : BaseController {

        private IClientManagementService _clientManagementService = null;

        public ClientController(IClientManagementService clientManagementService) {
            _clientManagementService = clientManagementService;
        }

        // this will work both for an HTTP GET (the returned value will got into the property
        // bag for the view to use) or for a JSON response (just like JsonReturnBinder - in fact 
        // that's where I got most of this code :-)
        [return: ViewModelReturnBinder]
        public MasterListOfClientsViewModel List(int page) {

            var results = _clientManagementService.GetClients(page);

            var viewModel = new MasterListOfClientsViewModel { 
                 Clients = results.Results,
                 PageNumber = page,
                 TotalPages = results.TotalPages
            };

            return viewModel;
        }
    }

    public class MasterListOfClientsViewModel {

        public MasterListOfClientsViewModel() {
            Clients = new List<Client>();
        }

        public IList<Client> Clients { get; set; }
        public int PageNumber { get; set; }
        public int TotalPages { get; set; }
    }

ViewModelReturnBinderAttribute.cs

// Decorate an action and just return a view model. It will be in the property bag if it's a page 
// GET, or just written to the response if it's an ajax request
public class ViewModelReturnBinderAttribute : Attribute, IReturnBinder {

        public void Bind(
                IEngineContext context, 
                IController controller, 
                IControllerContext controllerContext, 
                Type returnType, 
                object returnValue) {

            var raw = GetRawValue(returnType, returnValue, context.Services.JSONSerializer);
            
            // if this is an ajax request, cancel any view rendering
            if (IsAjaxRequest(context)) {
                controllerContext.SelectedViewName = null;
                context.Response.ContentType = "application/json, text/javascript";
                context.Response.Output.Write(raw);
            } else {
                controllerContext.PropertyBag.Add("_ViewModel", raw);
            }
        }

        private bool IsAjaxRequest(IEngineContext context) {

            if (context.Request.ContentType.Contains("application/json"))
                return true;

            if (context.Request.ContentType.Contains("text/javascript"))
                return true;

            return false;
        }

        /// <summary>
		/// Converts target object to its JSON representation
		/// </summary>
		/// <param name="returnType">Type of the target object</param>
		/// <param name="returnValue">The target object</param>
		/// <param name="serializer">The JSON serializer</param>
		/// <returns></returns>
		private string GetRawValue(Type returnType, object returnValue, IJSONSerializer serializer)
		{
			if (returnValue == null)
				return returnType.IsArray ? "[]" : "{}";

			return serializer.Serialize(returnValue);
		}

        class PropertyConverter : IJSONConverter {
            private readonly Type targetType;
            private List<PropertyInfo> properties = new List<PropertyInfo>();

            public PropertyConverter(Type targetType, string propertyNames) {
                this.targetType = targetType;

                foreach (string propertyName in propertyNames.Split(',')) {
                    PropertyInfo prop = targetType.GetProperty(propertyName);

                    if (prop == null) {
                        throw new MonoRailException("Failed to JSON serialize object. " +
                            "Property " + propertyName + " could not be found for type " + targetType.FullName);
                    }

                    properties.Add(prop);
                }
            }

            public void Write(IJSONWriter writer, object value) {
                writer.WriteStartObject();

                foreach (PropertyInfo info in properties) {
                    object propVal = info.GetValue(value, null);

                    writer.WritePropertyName(info.Name);
                    writer.WriteValue(propVal);
                }

                writer.WriteEndObject();
            }

            public bool CanHandle(Type type) {
                return type == targetType;
            }

            public object ReadJson(IJSONReader reader, Type objectType) {
                throw new NotImplementedException();
            }
        }
    }