I have a CKEditor textarea:
After reading this link http://docs.cksource.com/ckeditor_api/symbols/CKEDITOR.editor.html#setData, following code work for me.
CKEDITOR.instances.editor1.setData( '<p>This is the editor data.</p>' );
You need to use CKEditor's API instead.
Specifically, http://docs.cksource.com/ckeditor_api/symbols/CKEDITOR.editor.html#setData