public class ResultFormatAttribute : ActionFilterAttribute, IResultFilter
{
    void IResultFilter.OnResultExecuting(ResultExecutingContext context)
    {
        var viewResult = context.Result as ViewResult;

        if (viewResult == null) return;

        context.Result = new JsonResult { Data = viewResult.ViewData.Model };
    }
}

[ResultFormat]
public ActionResult Entries(String format)
{
    var dc = new Models.WeblogDataContext();

    var entries = dc.WeblogEntries.Select(e => e);

    return View(entries);
}

return Json(entries);

public class ResultFormatAttribute : ActionFilterAttribute, IActionFilter
{
    void IActionFilter.OnActionExecuted(ActionExecutedContext context)
    {
        context.Result = new JsonResult
        {
            Data = ((ViewResult)context.Result).ViewData.Model
        };
    }
}