How to validate and post data from dynamic form in ASP.NET MVC 2

I've just finished completing exactly the same task.

I chose to write a custom model binder for my dynamic form object. The model binder pulled out a bunch of prefixed form keys for hidden fields which contained some delimited meta data about the question (i.e IsRequired, QuestionType, QuestionId etc etc)

I'm using MVC3 but I think this should all work in MVC2.

I created a ModelBinder like:

public class DynamicFormModelBinder : IModelBinder
{
    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        // Create the object to be bound to (I had a kind of form object
        // with a simple list of answer objects
        DynamicForm form = new DynamicForm(new List<Answer>());

        HttpRequestBase request = controllerContext.HttpContext.Request;

        var keys = request.Form.AllKeys.Where(k => k.StartsWith("MyFormsKeyPrefix_Meta_"));
        foreach (var key in keys)
        {
            // Loop over each question's meta data. Metadata will always be present 
            // even if the user hasn't selected an answer as it's a hidden field

            // TODO: Split the meta data and pull out IsRequired, QuestionType etc

            // TODO: Get all the posted form values for the question (these values 
            //       will come from textboxes, dropdowns, checkboxes etc)
            //       Use a prefix like: MyFormsKeyPrefix_Answer_{QuestionId}
            //       textboxes & dropdowns will only ever have one value 
            //       but checkboxes could have multiple

            // TODO: If it's a mandatory question then ensure there is at least
            //       one posted value that is not an empty string

            // If there is a validation error then add it to the model state
            bindingContext.ModelState.AddModelError(key, "Field is required");

            foreach(var answerHtmlName in answerHtmlNames)
            {
                // TODO: Loop over each posted answer and create some kind of nice
                //       Answer object which holds the QuestionId, AnswerId, AnswerOptionId 
                //       and Value etc.


                // Add the answer to the forms answers list
                form.Answers.Add(answer);
            }
        }

        return form;
    }

}

I register the ModelBinder in Global.asax using the following:

ModelBinders.Binders.Add(typeof(DynamicForm), new DynamicFormModelBinder());

So, the action method that recieves the form post looks something like:

public ActionResult ProcessForm(DynamicForm form) {
    if(ModelState.IsValid) 
    {
        DynamicFormService.Process(form);

        return RedirectToAction("TheHttpGetAction");
    }
    return TheHttpGetAction();
}