My dirty solution for so far.

I extend the WebDateChooserProvider class and override the method bool CanEditType(Type t).

This method return now true if it's a datetime field, nullable datetime field or a string. Why the string is necessary? I don't know but it works. When by a string type return false the grid will be display an error if I bind an empty list. I use linq for my database connection and I cast the result recordset to a List<Data model object>.

public class NullableDateTimeEditorProvider : Infragistics.Web.UI.GridControls.WebDateChooserProvider
    {
        protected override bool CanEditType(Type t)
        {
            if (t.Name.Contains("Nullable"))
            {
                return t.FullName.Contains("System.DateTime");  // Nullable datetime support!
            }
            else if (t.Name.Contains("String"))
            {
                return true;                                    // Strings also true. Why? Read the description
            }
            else
            {
                return base.CanEditType(t);                     // Otherwise check the base method
            }
        }
    }