/** Selecciona/deselecciona todos los checkboxes the un formulario */
function checkUncheckAll(theElement) {
    //documentation for this script at http://www.shawnolson.net/a/693/
    var theForm = theElement.form, z = 0;
    while (theForm[z].type == 'checkbox' && theForm[z].name != 'checkall') {
        theForm[z].checked = theElement.checked;
        z++;
    }
}