jquery.mailchimp_submit.js

// Form tag will look something like this by default:
// <form action="http://xxxxx.us#.list-manage1.com/subscribe/post?u=xxxxx&id=xxxx" method="post" ... >

// change it to look something like this
// <form action="http://xxxxx.us#.list-manage1.com/subscribe/post-json?u=xxxxx&id=xxxx&c=?" method="get" ... >

$(document).ready( function () {
    var $form = $('form');

    if ( $form.length > 0 ) {
        $('form input[type="submit"]').bind('click', function ( event ) {
            if ( event ) event.preventDefault();
            // validate_input() is a validation function I wrote, you'll have to substitute this with your own.
            if ( validate_input($form) ) { register($form); }
        });
    }
});

function register($form) {
    $.ajax({
        type: $form.attr('method'),
        url: $form.attr('action'),
        data: $form.serialize(),
        cache       : false,
        dataType    : 'json',
        contentType: "application/json; charset=utf-8",
        error       : function(err) { alert("Could not connect to the registration server. Please try again later."); },
        success     : function(data) {
            if (data.result != "success") {
                // Something went wrong, do something to notify the user. maybe alert(data.msg);
            } else {
                // It worked, carry on...
            }
        }
    });
}