This is my custom model binder. I have my breakpoint set at BindModel but does not get fired with this controller action:

public ActionResult Create(TabGroup tabGroup)

...

public class BaseContentObjectCommonPropertiesBinder : DefaultModelBinder
{
    public new object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (controllerContext == null)
        {
            throw new ArgumentNullException("controllerContext");
        }
        if (bindingContext == null)
        {
            throw new ArgumentNullException("bindingContext");
        }

        BaseContentObject obj = (BaseContentObject)base.BindModel(controllerContext, bindingContext);
        obj.Modified = DateTime.Now;
        obj.Created = DateTime.Now;
        obj.ModifiedBy = obj.CreatedBy = controllerContext.HttpContext.User.Identity.Name;
        return obj;
    }

My registration:

// tried both of these two lines

ModelBinders.Binders[typeof(TabGroup)] = new BaseContentObjectCommonPropertiesBinder();
ModelBinders.Binders.Add(typeof(TabGroup), new BaseContentObjectCommonPropertiesBinder());