The problem: The controller don't get the data in textarea because FCK changes textarea for an iframe. The solution is  update the textarea content before submit.

I am using this helper in this way:

echo $html->textarea('Message/body', array("cols"=>30, "rows"=>10));
echo $fck->load('Message/body', 'Basic', 500, 200);

this is just to setup FCK, now the the ajax button

 echo $ajax->submit('Send message', array("url" => "/admin/messages/general",
                                         "update"=>"already",
                                         "before" => "MyFCKObject.UpdateEditorFormValue();",
                                         "loading" => "Element.show('charging');Element.hide('already')",
                                         "complete" => "Element.hide('charging');Effect.Appear('already')"
        ));

in the bottom of your view file:

echo $fck->fckAjax();

Voila! You can see and example.