after ajax form submit value from ckeditor textarea is not sent through post

You can force CKeditor to update the textarea value using:

for (instance in CKEDITOR.instances) {
    CKEDITOR.instances[instance].updateElement();
}

Also, you can use .serialize for the data - then you won't have to maintain the AJAX code if parameters change:

<script src="ckeditor/ckeditor.js"></script>
function saveArt() 
{
    for (instance in CKEDITOR.instances) {
        CKEDITOR.instances[instance].updateElement();
    }

    jQuery.ajax({
        type: 'POST',
        url: 'art_save.php',
        data: $("#art").serialize()
     });  
     return false; 

 }

for (instance in CKEDITOR.instances) 
{
    CKEDITOR.instances[instance].updateElement();
}

$.ajax({
    url: "controllers/c.publicidad.php?task=save",
    type: 'post',
    data: $("form[name='form']").serialize(),
    dataType: 'json',
    success: function(data) {
        if(data.idenvio != ''){
            $("form[name='frmPublicidad']").toggle("slow");
            $("#MessageSuscripcion").html(data.message);
        }
    }
});