AJAX Post of JavaScript String Array to JsonResult as List Always Returns Null?

I faced the same problem after updating to jquery 1.4.2. You can find the solution here (in the Ajax section).

Adding traditional : true in the ajax options should work.

$.ajax({
    type: "POST",
    traditional: true,
    url: "/Test/JSONTestAction",
    async: false,
    data: parms,
    dataType: "json",
    success: function(data) {

        // success
    }
});