gistfile1.htm

<form>
    <fieldset id="fields"></fieldset>
</form>

<script>
onload = function () {

    var fields = document.getElementById('fields'),
        frm = fields.parentNode;

    function updateCookie(checkbox) {
    }

    function addInput(value) {
        var inputs = document.getElementsByName('a_text'),
            add = true,
            div,
            i;

        for (i = inputs.length - 1; i >= 0; --i) {
            if (!/\S/.test(inputs[i].value)) {
                add = false;
            }
        }

        if (add) {
            div = document.createElement('div');
            div.innerHTML = 
                '<input type="checkbox" name="a" value=""> ' +
                '<input type="text" name="a_text" value="' + (value || '') + '">';

            attach(div);
            fields.appendChild(div);
        }
    }

    function attach(el) {
        var inputs = el.getElementsByTagName('input'),
            checkbox = inputs[0],
            text = inputs[1],
            last;

        checkbox.onclick = function () {
            updateCookie(checkbox, text);
        };

        text.onblur = function () {
            updateCookie(checkbox, text);

            addInput();
        };
    }

    addInput();
};